134 mr. darwin's work and 



lays down be correct. Take the case of the races which 

 are known to be produced by the operation of atavism 

 and variability, and the conditions of existence which 

 check and modify these tendencies. Take the case of 

 the pigeons that I brought before you : there it was 

 shown that they might be all classed as belonging to 

 some one of five principal divisions, and that within 

 these divisions other subordinate groups might be 

 formed. The members of these groups are related to 

 one another in just the same way as the genera of a 

 family, and the groups themselves as the families of an 

 order, or the orders of a class ; while all have the same 

 sort of structural relations with the wild Rock-pigeon, 

 as the members of any great natural group have with a 

 real or imaginary typical form. Now, we know that 

 all varieties of pigeons of every kind have arisen by a 

 process of selective breeding from a common stock, the 

 Rock-pigeon ; hence, you see, that if all species of ani- 

 mals have proceeded from some common stock, the gen- 

 eral character of their structural relations, and of our 

 systems of classification, which express those relations, 

 would be just what we find them to be. In other 

 words, the hypothetical cause is, so far, competent to 

 produce effects similar to those of the real cause. 



Take, again, another set of very remarkable facts, — 

 the existence of what are called rudimentary organs, 

 organs for which we find no obvious use, in the par- 

 ticular animal economy in which they are found, and 

 yet which are there. 



Such are the splint-like bones in the leg of the 

 horse, which I here show you, and which correspond 

 with bones which belong to certain toes and fingers 

 in the human hand and foot. In the horse you see 



