504 The Study of Animal Life part iv 



characters of animals may be hypothetically interpreted as 

 the result of use or disuse. Is the torpedo-like shape of 

 swift swimmers due to their rapid motion through the 

 water, do burrowing animals necessarily become worm-like, 

 has the giraffe lengthened its neck by stretching it, have 

 hoofs been developed by running on hard ground, are horns 

 responses to butting, are diverse shapes of teeth the results 

 of chewing diverse kinds of food, are cave-animals blind 

 because they have ceased to use their eyes, are snails lop- 

 sided because the shell has fallen to one side, is the 

 asymmetry in the head of flat fishes due to the efforts made 

 by the ancestral fish to use its lower eye after it had formed 

 the habit of lying flat on the bottom, is the woodpecker's 

 long tongue the result of continuous probing into holes, are 

 webbed feet due to swimming efforts, has the food-canal in 

 vegetarian animals been mechanically lengthened, do the 

 wing bones and muscles of the domesticated duck compare 

 unfavourably with those of the wild duck because the habit 

 of sustained flight has been lost by the former ? 



But these interpretations have not been verified ; they 

 are only probable. "It is infinitely easy," Semper says, 

 " to form a fanciful idea as to how this or that fact may be 

 hypothetically explained, and very little trouble is needed to 

 imagine some process by which hypothetical fundamental 

 causes — equally fanciful — may have led to the result which 

 has been actually observed. But when we try to prove by 

 experiment that this imaginary process of development is 

 indeed the true and inevitable one, much time and laborious 

 research are indispensable, or we find ourselves wrecked on 

 insurmountable difficulties." 



Not a few naturalists believe in the inherited effects 

 of functional change mainly because the theory is simple 

 and logically sufficient. If use and disuse alter the 

 structure of individuals, if the results are transmitted and 

 accumulate in similar conditions for generations, we require 

 no other explanation of many structures. 



The reasons why not a few naturalists disbelieve in the 

 inherited effects of functional change are (i) that definite 

 proof is wanting, (2) tliat it is difficult to understand how 



