fare of the species. In short, Darwin's views kept pace with the investigations of his day, and tended in the direction of restricting rather than widening the influence of natural selection. But, as Romanes, and especially Spencer, in his "Factors of Organic Evolution," have fully shown Darwin's position on this subject, I may pass over the detail.
Internal Conditions.—Physiological.—Genesis itself is the first and most fundamental of all causes of variation. The philosophy of sex may, indeed, be sought in this differentiation, as the accumulated qualities in separate entities when suddenly conjoined or commingled inevitably lead to aggregation and heterogeneity—in other words, to plasticity or capacity to vary. Genesis, as a fundamental factor in evolution, may be more intelligently considered under some of its subordinate phases, as heredity, physiological selection, sexual selection, primogenital selection, sexual differentiation, including philoprogeneity, hybridity, etc.
Heredity, as expounded by the ablest biologists, and as exemplified in life, is a puissant factor in evolution, and, though essentially conservative, must, through the marvelous power of atavism, tend to increase individual variability. The subject has been too well considered by Darwin and his followers to justify further discussion of it here. As a cause of variation, heredity must, however, have less and less influence as we go back in the scale of organized beings; for it can not well come into play in agamic or fissiparous reproduction, a fact which has given the abiogenisists one of their strongest arguments, since it is difficult to understand how, for instance, the monera of to-day could have descended without change from the primordial form.
Physiological Selection.—Physiological selection, as suggested by Mr. Catchpool and as expounded by Romanes, is undoubtedly a veritable factor in evolution, and, while giving us another link in the chain of evidence as to the causes of differentiation, lessens in but very slight degree the overwhelming force of the argument for natural selection. It adds, rather, an important element in the evidence therefor, and may be classed as a subordinate cause of differentiation. Romanes's theory is based upon the argument that differences, such as constitute varieties and species in their commencement, would not be preserved by natural selection unless useful, but would be lost again by cross-breeding with forms like the parent, and which had not varied, except upon some hypothesis like that of physiological selection. This could not be prevented except by migration. This difficulty is a general one, was argued by Darwin himself, and has been felt by all Darwinians. The reproductive organs are extremely variable, and sterility may occur not only between species, but between races and varieties, and often between individuals. Physiological selec-
