DARWIN ON THE ACQUIREMENT OF RIGIDITY. 41

rially lessens the difficulty of understanding the evolution of many complex organs, and abridges the time which must have elapsed during their formation. The relation, however, of nutrition to evolution will be subsequently considered*.

Organisms vary greatly in their susceptibility to undergo variation; some vary so slightly, and others so considerably under the same conditions, that one is constrained to believe that the amount of variability exhibited by any species is due to constitutional influences. Hence organisms may be spoken of as exhibiting various degrees of plasticity † or rigidity.

Mr. Darwin very justly attributes the greater part of the modification of species by natural selection to the gradual accumulation of slight, usually indefinite, variations. He does not, however, appear to recognize that plasticity, or the power to vary, may be increased as well as diminished by natural selection, although this latter fact is undoubtedly admitted by him; for he says, when speaking of highly modified organs occurring in species the allies of which have the same organs considerably less developed, "such organs must have gone through an extraordinary amount of * Chapter X.

+ Capable of being moulded, modelled, or fashioned to a purpose.

42 ON THE ACQUIREMENT OF PLASTICITY.

modification since the genus arose; and thus we can understand why they should often still be variable in a much higher degree than other parts; for variation is a long-continued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state"*

In the opinion of the author there is abundant evidence that plasticity, or the power to vary, is transmitted from generation to generation like any other hereditary character. The most variable individuals, if paired, probably produce the most variable offspring, just as the long slender individuals, when paired, produce long slender offspring. Every constitutional tendency is known to be hereditary to a greater or less degree. The greater variability of domesticated animals and cultivated plants, as compared with wild forms, is probably due to the unconscious selection of the most plastic individuals. Those which vary most either definitely or indefinitely (that is, those which are most readily influenced by external or internal conditions) are the most plastic. The variation of such is most distinctly marked; and they are therefore selected for breeding; hence the most plastic are unconsciously selected.

*Origin of Species, p. 132.

VARIATION UNDER NATURE.

43

It may be that the extreme increase of plasticity which would originate in this manner from the continual pairing of the most variable individuals (that is, those presenting extreme variations) causes the great delicacy of constitution so apparent in highly modified forms produced artificially. These are especially liable to give birth to malformed young; and it requires great care and attention to rear their offspring. Slight causes acting on an abnormally plastic organism produce changes which are incompatible with life or health; and the embryo is often so irregularly developed that it is incapable of living after separation from its parent. A single cross, probably by diminishing the plasticity of the organism, often improves the stamina of such breeds materially.

In nature such excessive plasticity is rarely, if ever, acquired; and this is probably due to constant crossing of the more by the less variable individuals, as well as to the fact, strongly insisted on by Mr. Darwin, that great deviations from the normal type could probably scarcely ever be useful in a complex animal, all the parts of which are, so to speak, nicely balanced against each other.

Just as selection of the most variable forms would apparently lead to increased plasticity, so the selection of the least variable would probably lead to rigidity of the organism. This would occur

44

RARITY A RESULT OF RIGIDITY.

whenever rigorous selection has perfected any part, and the conditions of life become fixed and constant. In this case the species would flourish as long as its surroundings remained unchanged; but extinction would probably follow any rapid or considerable change in its conditions of life, since, from the gradual elimination of plasticity, it would be little adapted to withstand even slight changes which require modification of structure.

Individuals belonging to any species which has not been subjected to changed conditions for long periods of time would tend to become rarer as soon as any change occurred, from loss of plasticity. Frequent crosses with other slightly different individuals, by introducing slightly different conditions, would, on the other hand, favour the occurrence of plasticity. This view accounts for the great benefit resulting from crossings with other individuals, and the greater variability of mongrel races. The vast number of peculiarities of organization which exist in nature to ensure cross fertilization in plants and hermaphrodite animals shows the high importance of its influence on the race.

Widely spread and common species are the most variable *; variability, as we have seen, is the direct result of changing conditions acting upon a plastic organization. Species are probably common * Darwin's Origin of Species,' p. 42.

VARIABLE ORGANS.

45

and widely spread from their plasticity; and this plasticity would be the result of frequent crossings between individuals slightly different from each other, and of continually changing conditions which have followed continuously upon those by which the species was first determined in its descent from a primitive type.

The species of large genera are more variable than those of small genera*; and this is probably to be accounted for in the same manner the genera are large and wide-spread because they are plastic.

In complex organisms the different organs of the body present very various conditions of plasticity; those which have been most rigorously selected under unchanged conditions vary least. As Mr. Darwin has pointed out, rudimentary organs, secondary sexual characters, as sexual ornaments and organs of offence and defence, present only in one sex, are extremely variable; such parts have not, at least in many cases, been subjected to the searching influence of natural selection.

Organs which are highly developed in one species, but less developed in all the allied species, as we have already seen, tend to become highly variable; such organs bear indications of having been recently developed by the natural selection of those * Darwin's Origin of Species,' p. 44.