(see page 347). And we have seen that the farther back we go toward the 

 one-celled stage, the more and more do these stages resemble corresponding 

 stages of other species. Thus in the life history of a mammal there are struc- 

 tures that suggest stages in the life history of birds and of fishes (see illustration, 

 p. 459). The larvae of different kinds of mosquitoes are more alike than the 

 larvae of mosquitoes and beetles; the larvae of insects in general are more 

 alike than the larvae of insects and crabs, and so on (see page 355). Now the 

 most reasonable explanation of such facts is the supposition that there is a 

 common (or similar) development just to the extent that organisms are "re- 

 lated" through having descended from common ancestors. 



Useless Structures Relationship is further inferred from the fact that 

 in plants and in animals certain organs persist through whole groups, although 

 they are quite useless from the point of view of adaptation. For example, the 

 whale develops legs that are never used, and the same is true of certain snakes. 

 The skeleton of many a bird shows distinct signs of fingers, or claws, among 

 the wing bones. The vermiform appendix (see illustration, p. 175) in man has 

 been interpreted as the lingering remains of an organ that developed and took 

 part in digestion in other backboned animals. It has no practical meaning in 

 the life of man today — except to make trouble sometimes. We can under- 

 stand such examples readily if we suppose that all plants and all animals are 

 related through having had common ancestors. No other theory that agrees 

 with all the facts has been suggested to explain such "vestigial" structures. 



Geographic Distribution We expect every group of organisms to ex- 

 pand its range just as far as conditions permit. And we rather expect a given 

 kind of situation to maintain one kind of population and a different kind of 

 situation to maintain a different kind of population. Yet when we examine 

 the distribution of species over the surface of the earth, certain curious facts 

 appear. 



Regions in every way similar, as to climate, soil, and so on, are inhabited 

 by totally different plants and animals. Thus the climate of Australia is 

 not very different from that of most of Europe and large parts of Africa, 

 Asia, North America and South America, yet Europeans who first came 

 to AustraUa found plants and animals that are not found in these other parts 

 of the world. Many such puzzling differences are found in comparing the 

 flora and fauna (plant and animal populations) of regions that are geograph- 

 ically similar. 



On the other hand, regions that are very different in climate, soil, and so 

 on, are occupied by plants and animals that are so much alike that we class 

 them in the same families. Thus goats and sheep, obviously related to each 

 other genetically, occur naturally in the Tropical Zones, as well as in the 

 Temperate Zones, and well up into the Arctic and Antarctic circles, Uving in 

 many kinds of surroundings. 



460 



