CHAP. IX. J HYBRIDISM. 219 



distinct, species, when inhabiting distant parts of the world and 

 living under considerably different conditions of life, yet often 

 retaining nearly the same instincts. For instance, we can under- 

 stand, on the principle of inheritance, how it is that the thrush 

 of tropical South America lines its nest with mud, in the same 

 peculiar manner as does our British thrush; how it is that the 

 Hornbills of Africa and India have the same extraordinary instinct 

 of plastering up and imprisoning the females in a hole in a tree, 

 with only a small hole left in the plaster through which the males 

 feed them and their young when hatched ; how it is that the male 

 wrens (Troglodytes) of North America build " cock -nests," to roost 

 in, like the males of our Kitty-wrens, a habit wholly unlike that 

 of any other known bird. Finally, it may not be a logical deduction, 

 but to my imagination it is far more satisfactory to look at such 

 instincts as the young cuckoo ejecting its foster-brothers, ants 

 making slaves, the larvae of ichneumonidse feeding within the 

 live bodies of caterpillars, not as specially endowed or created 

 instincts, but as small consequences of one general law leading to 

 the advancement of all organic beings, namely, multiply, vary, 

 let the strongest live and the weakest die. 



CHAPTER IX. 

 HYBRIDISM. 



Distinction between the sterility of first crosses and of hybrids Sterility 

 various in degree, not universal, affected by close interbreeding, removed 

 by domestication Laws governing the sterility of hybrids Sterility not 

 a special endowment, but incidental on other differences, not accumulated 

 by natural selection Causes of the sterility of first crosses and of hybrids 

 Parallelism between the effects of changed conditions of life and of 

 crossing Dimorphism and trimorphism Fertility of varieties when 

 crossed and of their mongrel offspring not universal Hybrids and 

 mongrels compared independently of their fertility Summary. 



THE view commonly entertained by naturalists is that species, 

 when intercrossed, have been specially endowed with sterility, in 

 order to prevent their confusion. This view certainly seems at 

 first highly probable, for species living together could hardly have 

 been kept distinct had they been capable of freely crossing. The 

 subject is in many ways important for us, more especially as the 

 sterility of species when first crossed, and that of their hybrid 

 offspring, cannot have been acquired, as I shall show, by the 

 preservation of successive profitable degrees of sterility. It is aa 



