218 LECTURE VIII. 



(in the case I have in view it was done by the interference of 

 man). The short legs become numerous, and constitute a 

 large proportion in the number of sheep of that region. We 

 now possess a variety which, by naturalists, is considered the 

 product of local influences. The variety is now described as 

 the short-legged, and in the museums placed side by side with 

 the long-legged sheep ; and the name of variety seems the 

 more justifiable, since, among the lambs produced by the short 

 legs, there are always some with longer legs, which seems a 

 relapse into the original type. 



In the above case, it was the advantage of the proprietor 

 to possess short-legged sheep unable to surmount the fences. 

 He therefore interfered ; he coupled his short-legged ram with 

 his shortest-legged descendants, purged his flock from all the 

 long legs, and thus he obtained, in the course of time, a short 

 legged race, which is now spread over North America. Now, 

 in the course of time the births of long-legged lambs have 

 become rare among the short-legged sheep, and the abnormity 

 has become permanent, so that man has first formed a variety, 

 and finally a race, — for races are defined constant varieties, 

 whose characters are permanently propagated. 



What man has done here, nature does everywhere. We 

 may consider every species, with its distinguishing characters, 

 as the product, to a great extent, of the influences acting upon 

 it. Every day in the life of an individual is a struggle for 

 existence. The individuals will be best developed where the 

 struggle is easy. The conditions of life vary for each species ; 

 hence each species will best thrive in one or several centres, 

 but wiU degenerate or become extinct in others. We usually 

 consider that the type of a species which is developed under 

 the most favourable conditions ; we consider as varieties or 

 races such forms as under less favourable conditions have de- 

 viated. The mussels peculiar to the seas of our temperate 

 zone become smaller, and are differently marked at the limits 

 of their region, be it north or south. They miss the con- 

 ditions requisite for their growth, and are no longer seen. 

 But among the mussels on the German and French coasts 

 there are some which increase in size as they move towards 



