something additional have to pass through more numerous stages than their ancestors; and those which lose something pass through fewer stages than their ancestors; and these processes are expressed by the terms 'acceleration' and 'retardation.'"*

It is clear, however, that we have here something more than acceleration and retardation of development in the ordinary sense of these words. It would be, therefore, more convenient to use the term "acceleration " for the condensation of the same series of developmental changes into a shorter period of time; "retardation" for the lengthening of the period in which the same series of changes are effected; and "arrested development" for those cases in which the young are born in an immature or embryonic condition. Whether there is any distinct tendency, worthy of formulation as a law, for organisms to acquire, as a result of protracted embryonic development, definite characteristics which their ancestors did not possess, I think very questionable. If so, this will fall under the head of the origin of variations.

That acceleration, in the sense in which I have used. the term, does occur as a variation is well known. "With our highly improved breeds of all kinds," says Darwin,† "the periods of maturity and reproduction have advanced with respect to the age of the animal; and in correspondence with this, the teeth are now developed earlier than formerly, so that, to the surprise of agriculturalists, the ancient rules for judging of the age of an animal by the state of its teeth are no longer trustworthy." "Disease is apt to come on earlier in the child than in the parent; the exceptions in the other direction being very much rarer." Professor Weismann contends that the time of reproduction has been accelerated through natural selection, since the shorter the time before reproduction, the less the number of possible accidents. We may, perhaps, see in the curious cases of

* Cope, "Origin of the Fittest," pp. 226, 125, and 297.
+ "Animals and Plants under Domestication," vol. ii. p. 313.

Ibid. p. 56.

reproduction during an otherwise immature condition, extreme instances of acceleration. The axolotl habitually reproduces in the gilled, or immature condition. Some species of insects reproduce before they complete their metamorphoses. And the females of certain beetles (Phengodini) are described by Professor Riley as larviform.*

Precocity is variation in the direction of acceleration, and that condensed development which is familiar in the embryos of so many of the higher animals may be regarded as the result of variations constantly tending in the same. direction. That there are fewer examples of retardation is probably due to the fact that nature has constantly favoured those that can do the same work equally well in shorter time than their neighbours. But there can be no doubt that, accompanying that fosterage and protection which is of such marked import in the higher animals, there is also much retardation. And as bearing upon the supposed law of variation as formulated by Messrs. Hyatt and Cope, it should be noted that this retardation or decreased rate of growth leads to the production of the more advanced descendant.

The Inheritance of Variations.

Given the occurrence of variations in certain individuals of a species, we have the alternative logical possibilities of their being inherited or their not being inherited. The latter alternative seems at first sight to be in contradiction to the law of persistence. Sir Henry Holland, seeing this, remarked that the real subject of surprise is, not that a character should be inherited, but that any should ever fail to be inherited.† Intercrossing may diminish a character, and sooner or later practically obliterate it: annihilate it at once and in the first generation it cannot. This logical view, however, ceases to be binding if we admit,

Nature, vol. xxxvi. p. 592.

Quoted from "Medical Notes and Reflections," 1855, p. 267, by Darwin, "Animals and Plants under Domestication," vol. i. p. 446.

with Professor Weismann, that variations may be produced in the body without in any way affecting the germ. It is also vitally affected if we believe that the hen does not produce the egg, though she may, perhaps, modify the eggs inside her; for the modification of the hen (i.e. the variety in question) may not be of the right nature or of sufficient strength to impress itself upon the germinal matter of the egg. We may at once admit, then, that acquired variations need not be inherited.

Passing to innate variations-variations, that is to say, which are the outcome of normal development from the fertilized ovum-must they be inherited, at any rate, in some degree? It seems to me that they must, on the hypothesis that sexual generation involves simply the blending or commingling of the characters handed on in the ovum or the sperm. The only cases where this would apparently fail to hold good would be where the ovum and the sperm handed on exactly opposite tendenciesa variation in excess contributed by the male precisely counterbalancing a variation in the opposite direction contributed by the female parent. Even here the tendency is inherited, though it is counterbalanced. On the hypothesis of "organic combination" before alluded to (p. 150), variations might, however, in the union of ovum and sperm, be not only neutralized, but augmented. If the variation be, so to speak, a definite organic compound resulting from a fortunate combination of characters in ovum and sperm, it might either fail altogether, or be repeated in an enfeebled form, or augmented in the offspring, according as the new conditions of combination were unfavourable or favourable.

Whether innate variations ever actually fail to be inherited, even in an enfeebled form, it is very difficult to say; for if this, that, or the other variation fail to be thus inherited, it is difficult to exclude the possibility of its being an acquired variation not truly innate. Certainly variations seem sometimes to appear in one generation, and not to be inherited at all. And, as we have seen, Mr.

Romanes appeals to a gradual failure of heredity, apart from intercrossing, to explain the diminution of disused organs.

That a variation strongly developed in both parents is apt to be augmented in the offspring is commonly believed by breeders. Darwin was assured that to get a good jonquil-coloured canary it does not answer to pair two jonquils, as the colour then comes out too strong, or is even brown. Moreover,* "if two crested canaries are paired, the young birds rarely inherit this character; for in crested birds a narrow space of bare skin is left on the back of the head, where the feathers are upturned to form the crest, and, when both parents are thus characterized, the bareness becomes excessive, and the crest itself fails to be developed."

On the whole, it would seem that variations may either be neutralized or augmented in inheritance; but the determining causes are not well understood.

Another fact to be noticed with regard to the inheritance of variations is that some characters blend in the offspring, while others apparently fail to do so. Mr. Francis Galton,† speaking of human characters, gives the colour of the skin as an instance of the former, that of the eyes as an example of the latter. If a negro marries a white woman, the offspring are mulattoes. But the children of a lighteyed father and a dark-eyed mother are either light-eyed or dark-eyed. Their eyes do not present a blended tint. Among animals the colour of the hair or feathers is often a mean or blended tint; but not always. Darwin gives the case of the pairing of grey and white mice, the offspring of which are not whitish-grey, but piebald. If you cross a white and a black game bird, the offspring are either black or white, neither grey nor piebald. Sir R. Heron crossed white, black, brown, and fawn-coloured Angora rabbits, and never once got these colours mingled in the same animal, but often all four colours in the same litter. He also

Darwin, "Animals and Plants under Domestication,” vol. i. p. 465.
"Natural Inheritance," p. 12.

Q

crossed "solid-hoofed" and ordinary pigs. The offspring did not possess all four hoofs in an intermediate condition; but two feet were furnished with properly divided and two with united hoofs.* Professor Eimer† has noticed that, in the crossing of striped and unstriped varieties of the garden snail, Helix hortensis, the offspring are either striped or unstriped, not in an intermediate or faintly striped condition.

These facts are of no little importance. They tend to minimize, for some characters at least, the effects of intercrossing. The variations which present this trait may be likened to stable organic compounds, which may be inherited or not inherited, but which cannot be watered down by admixture and intercrossing. It is well known that, in 1791, a ram-lamb was born in Massachusetts, with short, crooked legs and a long back, like a turn-spit dog. From this one lamb § the otter, or ancon, breed was raised. When sheep of this breed were crossed with other breeds, the lambs, with rare exceptions, perfectly resembled one parent or the other. Of twin lambs, even, one has been found to resemble one parent, and the second the other. All that the breeder has to do is to eliminate those which do not possess the required character. And very rarely do the lambs of ancon parents fail to be true-bred.

Now, it can scarcely fail that such sports occur in nature. And if they are stable compounds, they will not be readily swamped by intercrossing. It only requires some further isolation to convert the sporting individuals into a distinct and separate variety. Now, Darwin tells us that

* Darwin, "Animals and Plants under Domestication," vol. ii. p. 70. "Organic Evolution," Mr. Cunningham's translation, p. 76.

Darwin, "Animals and Plants under Domestication," vol. i. p. 104. § Similarly, from a chance sport of a one-eared rabbit, Anderson formed a breed which steadily produced one-eared rabbits (" Animals and Plants under Domestication," vol. i. p. 456). This is an example of asymmetrical variation. Variations are generally, but not always, symmetrical. Superficial colourvariations are sometimes asymmetrical. Gasteropod molluscs are nearly always asymmetrically developed. Among insects, Anisognathus affords an example of the asymmetrical development of the mandible. Our right-handedness is a mark of asymmetry.