PRESENT CRISIS IN EVOLUTIONARY THOUGHT 5 



persisted. ... A form is not a species until it has 'stood.' '' 

 (Science, Oct. 20, 1922, p. 448.) 



Sexaal (gametic) incompatibility as a criterion of specific 

 distinction, presupposes the bisexual or biparental mode of 

 reproduction, namely, syngamy, and is therefore chiefly ap- 

 plicable to the metista, although, if the view tentatively pro- 

 posed by the protozoologist, E. A. Minchin, be correct, it 

 would also be applicable to the protista. According to this 

 view, no protist type is a true species, unless it is maintained 

 by syngamy (i.e. bisexual reproduction) — "Not until syngamy 

 was acquired," says Minchin, "could true species exist among 

 the Protista." ("An Introduction to the Study of the Proto- 

 zoa," p. 141.) 



To return, however, to the metista, the horse (Equus cabal- 

 liLs) and the ass (Equus asinus) represent two distinct species 

 under a common genus. This is indicated by the fact that the 

 mule, which is the hybrid offspring of their cross, is entirely 

 sterile, producing no offspring whatever, when mated with 

 ass, horse, or mule. Such total sterility, however, is not essen- 

 tial to the proof of specific differentiation; it suffices that the 

 hybrid be less fertile than its parents. As early as 1686, 

 sterility (total or partial) of the hybrid was laid down by 

 John Ray as the fundamental criterion of specific distinction. 

 Hence Bateson complains that Darwinian philosophy fla- 

 grantly "ignored the chief attribute of species first pointed 

 out by John Ray that the product of their crosses is frequently 

 sterile in a greater or lesser degree." (Science, Jan. 20, 1922, 

 p. 58.) 



Accordingly, the sameness of type required in members of 

 the same species refers rather to the genotype, that is, the 

 sum-total of internal hereditary factors latent in the germ, 

 than to the phenotype, that is, the expressed somatic char- 

 acters, viz. the color, structure, size, weight, and all other 

 perceptible properties, in terms of which a given plant or ani- 

 mal is described. Thus it sometimes happens that two dis- 



