CHANGING ANIMALS 17 



great bulk of fishes, even of the crossopterygian fishes, stayed in the water, 

 living and dying as might be determined by stringency of conditions con- 

 fronting them. A few were the pioneers into the new environment out- 

 side the water. 



As Simpson (1953) especially has pointed out, rates of evolutionary 

 change vary greatly, from animal to animal and from time to time. We 

 may be sure that the "chosen few" among crossopterygian descendants 

 in the Devonian were in a highly unstable condition as regards adaptation. 

 At first they must have been barely able to meet requirements of life in 

 the new environment; life must have been a ''nip and tuck" aff'air. Un- 

 der such precarious conditions any slight improvement might have made 

 an important contribution to survival and hence have been favored by 

 natural selection. This fact, together with the small numbers of individuals 

 involved, would have been conducive to rapid evolution. (The influence of 

 numbers upon rates of evolutionary change is discussed in Chapters 19, 

 20, and 21.) Consequently the shift from water to land probably occurred 

 quickly, in terms of geologic time. 



One reason for mentioning here the small numbers of transitional forms 

 and the brief span of the world's history in which they lived is to point 

 out that these facts explain in large measure why we seldom find fossils of 

 actual transitional forms between one major group of animals (such as 

 fishes) and another major group (such as amphibians). Transitional forms 

 are so seldom found, in fact, that one school of thought claims that they 

 never existed, that one group arose from another by one sudden change 

 ("systemic mutation" of Goldschmidt, 1940). This idea has been ex- 

 pressed by the striking statement: "The first bird hatched from a reptile's 

 egg." It seems more likely, however, that Simpson is correct in postulating 

 that transitional forms did occur but that they were so relatively few in 

 number and occurred during such a brief interval of geologic time that 

 chances of finding fossils of them are small. Moreover, in the following 

 chapters we shall note examples of transitional forms whose fossils have 

 been discovered. Chapter 7 presents additional information concerning 

 reasons why the geologic record is incomplete. 



Potentiality plus Opportunity 



We may appropriately mention at this point an erroneous idea prev- 

 alent among many people who know little about evolution. This is the 

 notion that if evolution is a fact all animals must be constantly tending 

 to become "higher" animals, or, in its most exaggerated form, that all 

 animals must be tending to become man. One sometimes hears the argu- 



