POLYMORPHISM. It now appears that the poly morphism of the social insects is due to the nature and amount of food. The existence of worker ants and bees, whose characteristics arc not inherited from their parents, has been a stumbling-block to the theory of descent. As Darwin states it: "The difficulty lies in under standing how such correlated modifications of structure could have been slowly accumulated by natural selection"; the "acme of the diffi culty" being "the fact that the neuters of several ants differ, not only from the fertile females and males, but from each other, sometimes to an al most incredible degree, and are thus divided into two or even three castes." The castes, moreover, do not commonly graduate into each other, but are perfectly well defined; being as distinct from each other as are any two species of the same genus, or rather as any two genera of the same family. It has also been found by Wheeler that in a Texan ant the several castes are at first all alike, the remarkable differences between the large' and small-headed workers being due to differences in the amount and nature of the food.
It is well known that the larvae of the worker honey-bees are fed with much less nutritious food than those of the queens, which are fed on 'pap' or 'bee-milk,' a highly nitrog enous food, which has apparently a singular power of developing the reproductive glands. The white ants (Termitiffi•) are remarkable for the polymorphism of the species, there be ing in one kind eight castes. among them workers with small heads, and others with large heads, and soldiers of two east es, i.e. smell-headed and large-headed. It has been found by Grassi that all these castes are horn alike, and that the differences between the castes are chiefly due to the varying nature of the food, and have nothing to do with heredity. The small-headed forms have a scanty diet, live on refuse matter, and even eat their own excrement, this being used in the construct ion of their galleries. The sol diers live on sick or disabled companions. The young are fed only with saliva. From these facts it appeals that the amount and nature of the food is the chief muse of the wonderful differ entiation of the castes of the social insects; be sides this there are the results of the division of labor in the community, use and disuse, together with specialization of labor, arising from the varied life of the populous colony.
Difference in the amount and nature of the food, involving low or under and high feeding, results in a discrepancy in the number of individ uals of either sex.
It is now clear that a preponderance in the number of females is the result of high or better feeding. Yung experimented on tadpoles and increased the proportion of females as the diet was improved. In the first brood, by feeding one set with beef, the percentage of females rose from 54 to 78; in the second set, fed with fish, it rose from 61 to SI ; while in the third lot, when the especially nutritious flesh of frogs was supplied, •the percentage rose from 56 to 92, i.e. in the last ease the result of high feeding was that there were 92 females to S males.
The result of Diising's experiments with sheep leaves little doubt that abundant moisture and food tend to the production of females, while high temperature produces males. The heavier well-fed ewes brought forth ewes, while the lighter, under-feel ewes gave birth to males. Caron divided a flock of 300 ewes into two equal parts, of which one-half were extremely well fed and served by two young rains, while the others were served by two mature rams and kept poorly fed. The proportion of ewe lambs was GO per cent. and 40 per cent., respectively.
UsE AND DISUSE AS FACTORS. Thus far we have considered the action of those factors which are concerned rather with the origination of varieties and species than of higher types; with the causes of specific variation rather than of the formation of genera, families, orders, classes, and branches or phyla. While gravity, light, and the allied factors evidently come into play in mo•phogenesis, the inquiry arises how the higher categories of organic forms originated. This must mainly have been accomplished through change of environment, inducing new needs, the formation of new habits, or change of function, all operating together and resulting in adapta tions to the new' mode of life. In all this the principle of use and disuse plays a most im portant part. See DIST:SE.