704 GENETICS AND EVOLUTION 



before being exposed to it. Let us suppose that a mutation occurs 

 wliich causes both eyes of a fish to develop on the same side of the 

 skull. If the fish continues in its old habits this will be a definite 

 handicap. But if it changes its mode of life and lies on its side at the 

 bottom of the sea and grubs in the mud for food, the new arrangement 

 will be advantageous. This mutation actually has occurred in the 

 flounder and sole. 



The theory of preadaptation provides a reasonable explanation for 

 occurrences such as the evolution of land forms. For example, in a species 

 of fish inhabiting a lake or river of the Devonian Period, some 350,- 

 0()(),()0() years ago, mutations may have occurred for the formation of 

 primitive lungs and for changing the fan-shaped fins to sturdier, limblike 

 fins with a fleshy lobe at the base. These changes would have had no 

 survival value for the fish as long as it lived in a lake or stream. Indeed, 

 the loss of the fan-shaped fins might have been deleterious, by interfering 

 with its ability to swim rapidly. The Devonian Period was one of violent 

 climatic changes, with seasons of drought alternating with rainy seasons. 

 As the streams dried up during one of the periods of drought, the water 

 became stagnant and lacked enough oxygen for the gills to function 

 properly in respiration. The fish with lungs, however, could come to 

 the surface, take a gulp of air, and obtain oxygen by diffusion across the 

 membrane lining the lungs. When the pond or stream dried up com- 

 pletely, he could use his sturdy, lobe-shaped fins to help squirm across 

 the intervening land to some other stream. Some process such as this 

 probably began the conquest of the land by vertebrates. Certainly the 

 first vertebrates to venture out of the water onto land were not seeking 

 air, for they and their ancestors had lungs and they could get air by 

 coming to the surface of the water. It is unlikely that they were fleeing 

 from predators, for they were among the largest animals of the time. 

 Since they ate other fish, and the only food on land consisted of plants 

 and insects, it can hardly be supposed that they were looking for food. 

 We are led to the somewhat paradoxical conclusion that the first verte- 

 brates to come out on land may have been looking for water, for their 

 own stream had just dried up! 



302. Mutations, the Raw Material of Evolution 



The Dutch botanist Hugo de Vries, one of the three rediscoverers 

 of Mendel's laws, was the first to emphasize the importance in evolution 

 of sudden, large changes rather than the gradual accumulation of many 

 small changes postulated by Darwin. In his experiments with plants, 

 such as the evening primrose, de Vries found that many unusual forms, 

 which differed markedly from the ancestral wild plant, appeared and 

 bred true thereafter. He applied the term mutations to these sudden 

 changes in the characteristics of an organism (earlier breeders had 

 called them "sports"). Darwin had observed such changes, but thought 

 they occurred too rarely to be of importance in evolution. Darwin be- 

 lieved that these sudden changes would upset the harmonious relations 

 between the various parts of an organism and its adaptation to the 



