NATUR/VL SELECTION 251 



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. 



darwin's summary of his answer to the difficulty as to the inability of 

 natural selection to account for the fact that species when crossed 

 are sterile or produce sterile offspring, whereas when v/vrieties 

 are crossed their fertility is unimpaired 



First crosses between forms", sufficiently distinct to be ranked as 

 species, and their hybrids, are very generally, but not universally 

 sterile. The sterility is of all degrees, and is often so slight that the 

 most careful experimentalists have arrived at diametrically opposite 

 conclusions in ranking forms by this test. The sterility is innately 

 variable in individuals of the same species, and is eminently suscept- 

 ible to the action of favorable and unfavorable conditions. The degree 

 of sterility does not strictly follow systematic affinity, but is governed 

 by several curious and complex laws. It is generally different, and 

 sometimes widely different in reciprocal crosses between the same two 

 species. It is not always equal in degree in a first cross and in the 

 hybrids produced from this cross. 



In the same manner as in grafting trees, the capacity in one species 

 or variety to take on another, is incidental on differences, generally 

 of an unknown nature, in their vegetative systems, so in crossing, the 

 greater or less facility of one species to unite with another is incidental 

 on unknown differences in their reproductive systems. There is no 

 more reason to think that species have been specially endowed with 

 various degrees of sterility to prevent their crossing and blending in 

 nature, than to think that trees have been specially endowed with 

 various and somewhat analogous degrees of difficulty in being grafted 

 together in order to prevent their inarching in our forests. 



The steriUty of first crosses and of their hybrid progeny has not 

 oeen acquired through natural selection. In the case of first crosses 

 it seems to depend on several circumstances; in some instances in 

 chief part on the early death of the embryo. In the case of hybrids, 

 it apparently depends on their whole organization having been dis- 

 turbed by being compounded from tv/o distinct forms; the sterility 

 being closely allied to that which so frequently affects pure species, 

 when exposed to new and imnatural conditions of life. He who will 

 explain these latter cases will be able to explain the sterility of hybrids. 

 This view is strongly supported by a parallehsm of another kind: 

 namely, that, firstly, slight changes in the conditions of life add to tbi? 



