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larger in the young than in the adults, in some species at least *.
With regard to the heredity of such changes, the weight of evidence is certainly in its favour. Brown Séquard found that mutilations tended to be inherited; and there is little doubt that diseases due to malnutrition are transmitted to the offspring.
We may well believe, therefore, that altered conditions of nutrition are adapted to function, since the organ is nourished in a direct ratio to the work done—that such changes are hereditary to a high degree, and would tend to produce adapted varieties, or varieties suited to meet the contingencies of their mode of life—that increased functional activity would induce variability by increasing the amount of nutriment afforded to the active part; and hence both adaptive variation and minute indefinite variations would be occurring at the same time, thus, guided by the powerful agency of natural selection, hastening the perfection of a functionally active part or organ.
* Packard, · Nature,' vol. v. pp. 484, 485.
82 INCREASE OF THE LOWEST FORMS OF LIFE.
THE RELATION OF NUTRITION TO
The different opinions which have been held by thinkers concerning the relation of the functions of nutrition and reproduction are not a little remarkable. From the most elementary consideration of reproduction in its simplest form, it is apparent that its relation to nutrition is direct and unequivocal (p. 50). The protoplasmic substance in the lowest forms of life divides when it increases beyond a certain size; and this process is repeated whenever sufficient nutriment has been assimilated.
It is true that when the supply of nourishment falls short, as at the approach of winter, such an organism frequently becomes encysted (that is to say, surrounded by an envelope of excreted material) and breaks up into a multitude of minute reproductive bodies; so that it may appear on a very superficial view as if starvation hastened the process of multiplication. It must be remembered, however, that this is only a temporary condition, as each of the reproductive bodies will require exactly as much nourishment before it is itself capable of
reproduction as if it had been segregated from the parent mass in its more developed or larger condition; hence all that can be said upon this subject is that any temporary failing of food-material hastens the multiplication of the species only by accelerating the separation of the young from the parent. Such a separation amongst the lower forms of life is undoubtedly highly advantageous as a means of dispersion, by temporarily augmenting the number of organisms produced, although this temporary increase is undoubtedly in a direct ratio to the amount of nourishment afforded to the parent by surrounding conditions.
Similar arguments, however, have been illogically applied to the case of the higher animals by thinkers of very considerable pretensions; and thus the most serious' misconceptions have originated. It has been supposed by Mr. Doubleday (and many have followed him) that the rapid increase of animals and plants is checked by abundant nutrition. The established fact is brought forward that the reproduction of some plants by seed is favoured by a sterile soil; and it is asserted further, by the advocates of these views, that the fattest and best-fed individuals amongst animals produce the fewest offspring.
A careful consideration of the facts, however, will at once dissipate this delusion.
There can be no doubt that the tendency of diminished nutrition is to separate the young from its parent in the earliest possible condition of development in which it is capable of survival. We have seen that the breaking up of the encysted Amoeba favours greatly its dispersion and therefore its means of obtaining food ; it is therefore of the highest advantage to the species, and will surely tend to occur even when food is more abundant, as those individuals which are most prolific will have the best chance of survival.
In plants two forms of reproduction exist, buds and seeds. The buds usually remain attached to the parent; and when the amount of nourishment is extreme the formation of buds and leaves continues with great rapidity. Any cause checking further increase in this direction, either by the proportion of the roots to the leaves and new growing buds becoming diminished or from deficiency of foodmaterial in the soil, tends at once to induce the separation of seeds from the parent. These seeds are undoubtedly modified buds, and, like the minute reproductive particles of the Amoeba, serve to disseminate the species at a less cost to the parent organism. As this means of increase, however, is highly advantageous, it tends to become developed independently of the amount of food supplied.
In the higher forms of animal life the case is very different. Each race has already in them acquired the tendency to separate young at as early a period of development as is consistent with their survival; and the young, or ova, are constantly separated in each kind at nearly the same stage of development ; hence all the young, in any species, require an equal share of nutriment for their production. In this case the food-material is apparently shared between the offspring and the parent, and both are debilitated by any failing either in quantity or quality. It does not appear, from observation, that a slight diminution produces any difference in the number of the young which are born in the first generation; but as these are ill-nourished, fewer live to produce offspring; and the race degenerates and dies out if such conditions continue. On the other hand, a large supply of nourishment produces vigorous parents and vigorous offspring, which have a far better chance of reaching maturity.
. The argument that fat animals produce few young is invalid—first, because fattening depends rather on the nature of the food than on its quantity, and, secondly, because such animals are always kept under unnatural conditions to favour the deposition of fat. Remembering how slight the unnatural conditions are which will produce sterility,