640 THE PHYSIOLOGY OF REPRODUCTION 



For species belonging to distinct genera can rarely, and those 

 belonging to distinct families can never, be crossed. The parallelism 

 is, however, far from complete ; for a multitude of closely allied 

 species will not unite, or unite with extreme difficulty, whilst other 

 species, widely different from one another, can be crossed with 

 perfect facility. Nor does the difficulty depend on ordinary 

 constitutional differences, for annual and perennial plants, deciduous 

 and evergreen trees, plants flowering at different seasons, inhabiting 

 different stations, and naturally living under the most opposite climates, 

 can often be crossed with ease. The difficulty or facility depends 

 exclusively on the sexual constitution of the species which are 

 crossed; or on their elective affinity." Sixthly, cross sterility 

 between species may depend possibly in certain cases upon distinct 

 causes, such as deterioration due to unnatural conditions to which 

 the hybrid embryo may be exposed in the uterus, egg, or seed of the 

 mother. " Seventhly, hybrids and mongrels present, with the one 

 great exception of fertility, the most striking accordance in all other 

 respects; namely, in the laws of their resemblance to their two 

 parents, in their tendency to reversion, in their variability, and in 

 being absorbed through repeated crosses by either parent form." 

 It is obvious, however, that this last conclusion requires some 

 modification in the light of recent Mendelian research. 



Darwin maintains further that the cross fertility which exists 

 between the different varieties of various species of domesticated 

 animals, in spite of their great divergence in external appearance, 

 is the direct effect of domestication which eliminates the tendency 

 towards mutual sterility. In this way " the domesticated descendants 

 of species, which in their natural state would have been in some 

 degree sterile when crossed, become perfectly fertile together." Both 

 Darwin and Wallace lay stress upon the apparent existence of a 

 parallelism between crossing and change of conditions in so far as 

 these affect the power to reproduce. ' Slight changes of conditions 

 and a slight amount of crossing are beneficial ; while extreme changes, 

 and crosses between individuals too far removed in structure or 

 constitution, are injurious." l Furthermore, domestic animals are 

 less susceptible to the influences of changed conditions of existence 

 than wild animals, a fact which finds a parallel in the absence of 

 sterility between domesticated varieties of the same species. 



It is now clear that Darwin's views on in-breeding and on the results 

 of that process must be modified in the light of recent Mendelian 

 investigation, for, as already shown in an earlier chapter of this book, 

 there is a growing body of evidence to show that different degrees of 

 productivity down to complete sterility can be inherited as though 



1 Wallace (A. R), Darwinism, London, 1897. 



