RELATIONSHIP OF SPECIES. 255 



the terms "present" and "absent" are often used, as might be 

 shown by P and 0, thus : — 



PP+00=2P0 and 



P + 



Several investigators have pointed out a difference in crossing 

 various types ; that in some instances the characters follow the 

 usual Mendelian laws and do not blend but segregate, while in 

 other crosses the characters blend (as in equine Mules). 



"Whether non-segregating crosses will prove amenable to 

 Mendelian interpretation appears a matter of considerable 

 doubt. At present this difference in result appears of great 

 importance, and it has been suggested that it is the non- 

 segregating characters that one must look to for the conception 

 of species (De Vries*). I do not gather, however, that this 

 theory is accepted by Mendelians in general. 



The idea occurs to one that the same Mendelian factors are 

 carried through the generations irrespective of the fact that, let 

 us say, the Horse is becoming more "horsey," or the Ass more 

 asinine ; or whether it is a Horse or an Ass germ that carries 

 them. The colour of an equine Mule, for instance, is perhaps 

 acted on just as truly by the divergent parent factors as would 

 be the case were these combining factors both from Horse (or 

 Ass) parents ; they also no doubt affect such qualities as 

 immunity to certain diseases, sex determination, &c. There 

 appears, however, no factor for " horseness " or " assness," and 

 one therefore inclines to the conclusion that, as Mendelism does 

 not explain the intermediate forms resulting from the inter- 

 breeding of two diverse species, it supplies no evidence of the 

 evolution of these from a common ancestor. Should the formula 

 •for such crosses be discovered (if there be one), it would hardly 

 explain evolution. 



In reading the works of Professor Bateson t or Professor 



* ' Mutationstheorie.' f 'Mendel's Principles of Heredity,' 



