( 443 ) 



is physiologically so separated from the normal individuals that interero.'^siug is 

 excluded. Is this conclusion correct ? 



We have seen above that aberrational individuals occur again and again among 

 the normal specimens ; if therefore a physiological variety such as Romanes's theory 

 demands occurs once, a species is able to piroduce it again and again, as long as the 

 circumstances under which the species lives do not change. But to make the case 

 as favourable as possible we will assume that the normal individuals, which mav be 

 designated as iV, gave birth to the variety Fonly once, so that if all the specimens 

 of the variety were killed the variety would never reappear. Now, the offspring of 

 the variety Fwill in the first and following broods belong either all to the variety V, 

 or partly to the variety Fand partly to the normal form X. The first assumption 

 nobody can admit to occur, as this would mean that V is a species suddenlv branched 

 off from N, and as further we should have, instead of the explanation of the phenomenon 

 of sterility between species which it is the aim of the theory to give, merelv the 

 statement that species V has sprung up among the individuals of species N with 

 which it is infertile. Hence we have to do only with the second case, that part of 

 the offspring of V, at least part of the first brood after the origin of V, belong to X ; 

 these offspring of l^we will designate Nv in order to indicate that, though they are 

 normal, they are the immediate descendants of parents with varietal characters. Xv 

 cross inter se and with iV, not with V; which characters will the offspring of Kv 

 have, which itself, we repeat it, is the descendant of V? The offspring of Xv could be 

 either all identical with X'v (=iV), or partly identical with Xv and partly with V. The 

 first alternative means that each specimen of V produces two groups of individuals, 

 Vv and Xv, of which one group (Xv) never will give rise to a form similar to its 

 parent-form I", while the other group {Vv) produces both the parent- and grand- 

 parent-form. Although the assumption here made, that some of the offspring of one 

 specimen breed perfectly true (Xv), while the other offspring do not breed true, is 

 quite at variance with our experience, and therefore not acceptaljle to any naturalist, 

 we will nevertheless accept the assumption for the sake of argument. As V breeds 

 in and in, according to that assumption, it is argued that in the course of time V will 

 become so modified that it will also breed true like X and then be specifically 

 distinct. Apart from X'v and V here being already two species from the beginning, 

 one (Xv) breeding true, the other (V) producing per saltum in every generation 

 some specimens of Xv, an illustration will show at once that before the above- 

 mentioned modification is effected the variety V will altogether be swamped awa\'. 

 Let us assume (1) that in an isolated district two hundred specimens of a certain 

 species (one hundred of each sex) could find subsistence ; (2) that each female would 

 produce ten females and ten males ; (3) that at a given time one-tenth of the 

 specimens conformed to the variety V, i.e. were sterile with the other nine-tenths 

 (X), but fertile inter se ; and (4) that 80 per cent, of the offspring of V belonged 

 to V, the other 20 per cent, to X, which breeds true, — for convenience we shall take 

 into account only the females, — the numbers of the forms X and Fin the successive 

 broods would be as follows : — 



We see that the number of specimens of V diminishes, and that extinction of 



