THE PROBLEMS OF MORPHOLOGY. 13 



taining which are the original corresponding parts. Some 

 parts have become abnormally large; others have dwindled 

 or disappeared; and the relative positions of parts have often 

 been greatly changed. A bat's wing and a bird's wing are 

 analogous organs, but their frameworks are but partially 

 homologous. While in the bird the terminal parts of the 

 fore-limb do little towards supporting the wing, in the bat 

 the wing is mainly supported by enormously-developed termi- 

 nal parts. 



The effects of the struggle to survive, which here prompts 

 a simpler life with resulting degeneracy and there a different 

 life with resulting new developments, are far from being the 

 only causes of morphological obscurations. Fulfilment of 

 certain highly general requirements gives certain common 

 traits to plants of widely divergent classes ; and fulfilment of 

 certain other highly general requirements gives certain 

 common traits to animals of widely divergent classes. It was 

 remarked in the first volume ( 54/) that the cardinal distinc- 

 tion between the characters of plants and animals arises from 

 the fact that while the chief food of plants is universally 

 present the food of animals is scattered. Here it has to be 

 added that to utilize the universally distributed food the 

 ordinary plant needs the aid of light, and has to acquire 

 structures enabling it to get that aid; while the ordinary 

 animal, to utilize the scattered food, must acquire the struc- 

 tures needful for locomotion. Let us contemplate separately 

 the traits hence resulting in the vegetal world and the traits 

 hence resulting in the animal world. 



The familiar plantain meets the requirement by growing 

 stiff leaves enabling it to press down the competing grasses 

 around which would else shade it; but the great majority of 

 ordinary plants meet the requirement by raising themselves 

 into the air. Hence the need for a stem, and hence the fact 

 that plants of widely unlike natures similarly form stems 

 which, in achieving strength enough to support the foliage 

 and resist the wind, acquire certain adaptive structures hav- 



