92 THE ZOOLOGIST. 



or less than is apparent, and that although outward form has 

 altered, the germ-plasm has not done so to the same extent or 

 vice versa, i.e., as regards such practical powers as reproduction, 

 and the congeniality and similarity this suggests. 



One of the most puzzling cases that crop up is the lack of 

 breeding results from such apparently connected birds as 

 Carolina and Mandarin Ducks, as remarked by Finn, but we are 

 getting rather in advance of our argument. To return : the infer- 

 tility of agenesic hybrids is an unsolved problem, but it is possible 

 that it is not essentially different to the infertility of inbred species. 



Let us see what happens in this latter case, which is often 

 carried out thus. Man takes an extreme type, and keeps breed- 

 ing this type probably against any type that environment 

 would select as the carrier of the germ-plasm. Man then 

 produces what we may call an " uncongenial host," and the 

 uncongeniality of the host probably adversely affects the vigour 

 of the germ-plasm, thereby causing sterility. In short, Nature 

 refuses to be led down the wrong path. 



In the same way the hybrid bred from two very divergent 

 forms is probably an uncongenial host, far indeed from the two 

 forms Nature has been evolving : it probably has a twofold 

 diverse " drive," and although the two parent forms have 

 combined to produce the hybrid, and given so to speak the germ 

 into its keeping, the hybrid combination, either from the general 

 uncongeniality of the parental forms (to each other) that Nature 

 has been producing, or its "driving" in two opposite directions 

 through the influence of two different parental germ-plasmatic 

 "driving" powers, cannot unite in full vigorous combination 

 and undividedness to produce the perfect ripe filial germ-plasm; 

 the ovum and spermatozoa never, I believe, in sterile forms 

 becoming mature or "ripe." Admitting that the driving force 

 of these two evolutionary parental forms be a bio-chemical one, 

 the general explanation need not be different. 



As sterility arises from interbreeding, as well as from violent 

 crosses, it maybe that it is, as stated above, from much the same 

 cause, and if this were so it would be some evidence against the 

 suggestion that the latter is due to a difference in the architec- 

 tural unit as suggested by the school of Weismann. 



To follow out interbreeding mercilessly, this procedure must 



