24 ORGANIC EVOLUTION — PHYSICAL 



conditions fall within narrow limits. We see, for 

 instance, that though by discipline, aided by selective 

 breeding, one variety of horse has had its locomotive 

 power increased considerably beyond the locomotive 

 powers of other varieties; yet that further increase 

 takes place, if at all, at an uhappreciable rate. The 

 different kinds of dogs, too, in which different forms 

 and capacities have been established, do not show 

 aptitudes for diverging in the same directions at 

 considerable rates. In domestic animals generally, cer- 

 tain accessions of intelligence have been produced by 

 culture, but accessions beyond these are inconspicuous. 

 It seems that in each species of organism there is a 

 margin for functional oscillations on all sides of a mean 

 state, and a consequent margin of structural variations ; 

 that it is possible rapidly to push functional and struc- 

 tural changes towards the extreme of this margin in 

 any direction, both in an individual and in a race ; but 

 that to push these changes further in any direction, and 

 so to alter the organism as to bring its mean state up 

 to the extreme of the margin in that direction, is a 

 comparatively slow process." — Frinciples of Biology, vol. 

 i. p. 188. 



The domesticated dog is presumably descended from 

 one or more of the different wild varieties, or from their 

 relatives the wolves. Now, considering the length of 

 time dogs have been domesticated, and the severity of 

 the selection to which they have been subjected, our 

 largest dogs, the St. Bernards, Newfoundlands, mastiffs, 

 boarhounds, do not very greatly exceed wild dogs 

 or wolves in size, nor do our most intelligent dogs 

 greatly surpass them in intelligence ; but our smallest 

 dogs, some of them little bigger than rats, are very 

 much smaller, and some of our tame breeds are exceed- 

 ingly stupid. Clearly as regards dogs, we halve been 

 able to produce little evolution, but great retrogres- 

 sion. In some breeds there has undoubtedly been 



