290 SUMMARY. 



SUMMARY OF CHAPTER. 



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 experimental- 

 ists have arrived at diametrically opposite conclusions in 

 ranking forms by this test. The sterility is innately vari- 

 able in individuals of the same species, and is eminently 

 susceptible to action of favorable and unfavorable condi- 

 tions. 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 vege- 

 tative 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 en- 

 dowed 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 sterility of first crosses and of their hybrid progeny 

 has not been acquired through natural selection. In the 

 case of first crosses it seems to depend on several circum- 

 stances ; in some instances, in chief part on the early death 

 of the embryo. In the case of hybrids, it apparently de- 

 pends on their whole organization having been disturbed 

 by being compounded from two distinct forms ; the ster- 

 ility being closely allied to that which so frequently affects 

 pure species, when exposed to new and unnatural condi- 

 tions 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 parallelism of another kind; 

 namely, that, firstly, slight changes in the conditions of 

 life add to tli3 vigor and fertility of all organic beings ; and 

 secondly, that the crossing of forms which have been ex- 



