FROM THALES TO LAMARCK 13 



specially adapted to a life in the water, the unpaired fins dis- 

 appear with the departure from the moist element. In the 

 amphibians we find in 

 the larval stage a com- 

 plete rudimentary 

 fringe, though without 

 skeleton parts ; but it 

 becomes, as a rule, de- 

 generate in the adult 



stage. The paired FIG . I. THE LANCELET (Brancliiostvma 



extremities, however, lanceoiatum). 



reach in the higher 



vertebrates a more and more perfect development, for they help 



the animals to add to the conquest of the water that of the land 



and the air. They are therefore of special interest from the 



standpoint of comparative anatomy. 



All extremities of the higher vertebrates, however widely 

 they may differ in construction, may be traced back biogenetic- 

 ally to the so-called Ichthyopterygium, as we see it in the fins 

 of the lower shark-like fishes. Unequal growth of the single 

 skeleton parts and a considerable reduction in their numbers 

 transformed the Ichthyopterygium into the five-fingered extremity 

 characteristic of all vertebrates from the amphibians upwards. 

 But yet another change is demanded by the changed conditions 

 of life. As long as the extremities served as oars or rudders it was 

 an advantage that they developed into broad ' blades,' acting with 

 their entire surface. But on land the arms and legs are to act 

 as levers in carrying and moving the body. To do this, the 

 extremities must divide into small parts united together and 

 movable one upon the other ; in other words, they must form 

 joints. 



Everyone who has seen the skeleton of a bird will no 

 doubt object that in the denizen of the air nothing is to be seen 

 of a five-fingered extremity, and in most cases indeed nothing 

 but one finger. This objection is true, but it is obviously the 

 different function which causes the deviation from the rule and 

 produces frequently in the skeletons of adult animals considerable 

 reversion or a combination of the single skeleton parts, thus 

 making the affected parts better adapted to their special work. 



