118 CALIFORNIA ACADEMY OF SCIENCES. 



Mr. Romanes' side of the argument. He points out 

 that experiments in hybridizing are generally made 

 with very distinct species, and that even these are some- 

 times fertile, while if two closely related forms hybridize 

 they are said to be one species. He quotes Dean Her- 

 bert's experiments as proving that in several large 

 genera of plants not only are the hybrids often fertile 

 but sometimes more so than the parent stock, whence 

 he concludes "that the sterility or fertility of the 

 offspring does not depend upon original diversity of 

 stock; and that if two species are to be united in a 

 scientific arrangement on account of a fertile issue, the 

 botanist must give up his specific distinctions generally, 

 and intrench himself within genera." 



" Really close species," says Mr. Wallace, " which have 

 probably originated by one remove from a common an- 

 cestor have never yet been crossed in large numbers and 

 for several generations, under appropriately natural con- 

 ditions, so as to afford any reliable data. The mere fact 

 that not only animals of distinct genera, but even those 

 classed in distinct families as the pheasant and the 

 black grouse sometimes produce hybrid offspring in a 

 state of nature, is itself an argument against there being 

 any constant infertility between the most closely allied 

 species, since if that were the case we should expect the 

 infertility to increase steadily with remoteness of descent 

 till when we came to family distinctions absolute sterility 

 should be invariable." 



To these criticisms of Mr. Wallace, Mr. Romanes 

 replied in an article in the Nineteenth Century,* enti- 

 tled Physiological Selection. He freely grants that Mr. 

 Wallace has proved that some specific characters are 

 useful, but insists that this is not sufficient to prove that 

 all points of specific difference are so. The theory of 



* xxi, pp. 59-80. 



