In the earliest forms of propagation, the younger organism is a true copy of that from which it springs, the trifling differences being due, as Schmankewicz has shown, to outer influences. The differences of male, female, worker, and soldier are due to such outer influences. The differences in the younger organism, where propagation takes place through copulation, may be explained by the mixture of types, through which, by action and reaction, some qualities are intensified, while some others become latent or are entirely destroyed. To these mutual influences are to be added such as come from without, especially those of warmth, and of quantity and quality of food. Under too great an increase in temperature, the young organism may even be destroyed, the process of assimilation not being able to keep pace with it. Those variations which have led to the development of existing forms, that is, which were favorable to life, are chiefly such as could be brought about by relative or absolute increase of assimilation. This is true of mental, as well as of physical, qualities. It is a fact established without doubt, that the most common and most widely distributed species show the greatest variability, and that those species, on the contrary, which are now rare, although they were, perhaps, at earlier periods, the most common and extremely variable, vary, at the present date, the least of all. Following Darwin, one generally draws the conclusion that the severity of the struggle for existence favors the formation of varieties. For, it is said, the most common species fight the severest battle with one another, while the scanty representatives of rarer species come the least into competition and continue unchanged. But this theory is, in two ways, erroneous. In the first place, no attention is paid to the fact that a rare species may be exposed to a severe struggle against another species for the same nourishment, while a common species may, on the other hand, be exposed to no such struggle, and, supporting life from a generous supply of food, be subjected to but slight pressure. The conception of the Darwinians means nothing more or less than that the individuals of a species vary the more, the less favorable the conditions of nourishment; and this cannot be conceded. Again, the fact is to be taken into consideration, that the species at present common must have passed through a favorable period in which food was so plentiful that it not only afforded an abundance to individuals past the dangers of infancy and youth, but allowed, in addition, the existence of an ever-increasing number of individuals. And it is this period of increase, of abundance, not a period of struggle, which has developed the variations we now have before our eyes. In the same manner one must conclude, with regard to the rarer species, that the formerly existing numerous varieties were destroyed during the period of decline, that is, of overpowering pressure.