174 Darwin, and after Darwin. 



by several of these gentlemen, " I do not thank you for your 

 mules." Now, although naturalists have travelled far and 

 learnt much since those days, it appears to me that a modern 

 evolutionist might still turn to the horticulturist with the same 

 words. For assuredly he has no reason to thank the 

 horticulturist for his mules, until he has found a satisfactory 

 answer to the question why it is that natural species differ so 

 profoundly as regards their capacity for hybridizing. 



Advance on Herherfs Position. — If it be said that all my 

 work amounts to showing what Herbert said long ago— viz. 

 that the only true or natural distinction between organic types 

 is the sexual distinction — I answer that my work does much 

 more than this. For it shows that the principle of sterility 

 is the main condition to the differentiation, not merely of 

 species and genera, but also to the evolution of adaptations 

 everywhere, in higher as well as in lower taxonomic divisions. 

 Moreover, even though naturalists were everywhere to consent 

 to abandon specific designations, and, as Herbert advises, to 

 "entrench themselves behind genera," there would still re- 

 main the facts of what are now called specific differences (of 

 the secondary or morphological kind), and by whatever name 

 these are called, they alike demand explanation at the hands 

 of the evolutionist. 



Fritz Muller on Cross-infertility. — Fritz Mfiller writes, 

 " Every plant requires, for the production of the strongest 

 possible and most prolific progeny, a certain amount of 

 difference between male and female elements which unite. 

 Fertility is diminished as well when this degree is too low 

 (in relatives too closely allied) as when it is too high (in 

 those too little related)." Then he adds, as a general rule, 

 " Species which are wholly sterile with pollen of the same 

 stock, and even with pollen of nearly allied stocks, will 

 generally be fertilized very readily by the pollen of another 

 species. The self-sterile species of the genus Abutilon, 



