Chap. 38 



ORGANIC EVOLUTION CONSERVATION 



783 



Fig. 38.4. Convergent evolution {upper) by a fish, the shark; (center) by a 

 reptile (ancient Icthyosaurus); (bottom) and a mammal, the dolphin. They all live 

 or did live in the sea and all are fish-shaped although only distantly related. (Cour- 

 tesy, Moody: Introduction to Evolution. New York, Harper & Bros., 1953.) 



more than 200 million years ago. Certain of these ancient animals had offspring 

 that started side lines of descent. Some of these prospered and others disap- 

 peared. Those in the main lines lived on in uneventful safety as we see them 

 now. 



Nonadaptive Trends. These are tendencies for certain characteristics to keep 

 developing until they become useless or dangerous. Great increase in size is 

 one of these. Growth with increase in size is universal in living organisms. It 

 usually reaches a slightly variable limit evidently an adaptation for the plant 

 or animal and this is repeated generation after generation. We think of a mouse 

 of one size, a horse of another. In contrast to this was the size of the dinosaurs, 

 with Brontosaurus, 75 feet long, hazardous especially for land animals. There 

 were other causes for their extinction, but giantism must have been an impor- 

 tant one. The heavy, multibranched antlers of deer are claimed as nonadaptive 

 features. In connection with adaptation, as with nonadaptation, it is reaUzed 

 that many structures are useless when they begin to develop and are not large 

 enough to be selected by the environment till long afterward. In "The Origin 

 of Species" Darwin pointed out that nonadaptation was an unexplained diffi- 

 culty in the working of natural selection in evolution. 



