8 INTRODUCTION TO EVOLUTION 



of them, and what was their relationship to modern animals? According 

 to one point of view, formerly widely held, they became extinct, leaving no 

 descendants. Perhaps widespread calamities (such as floods) efi'ected 

 wholesale removal of these ancient animals. And perhaps they were then 

 replaced either by new animals especially created for the purpose or by 

 animals that migrated in from regions of the earth untouched by the 

 catastrophe in question. This theory of catastrophism was prevalent among 

 biologists of past centuries. The eminent French biologist, Cuvier, whose life 

 spanned the close of the eighteenth century and the first part of the nine- 

 teenth, was one of its most powerful exponents. 



We note that according to the theory of catastrophism the "new" ani- 

 mals inhabiting a given region after a catastrophe would not be the de- 

 scendants of the "old" animals formerly found in the region. They would 

 be fresh creations, created either in the region in question or elsewhere. 

 This idea stands in direct contrast to the idea of organic evolution, which 

 holds that the "new" animals are modified descendants of certain of the 

 differing animals that formerly existed, in that region or some other. Not 

 that all the old animals left modified descendants; far from it. Evidence 

 indicates that only a small minority had that capability. The rest be- 

 came extinct without issue. 



The Changing World 



Returning to our original question concerning the fact of changes in ani- 

 mals, we may note that we should expect such change even if the geologic 

 record did not afford a direct testimony of it. It is a truism that change is 

 the only unchanging aspect of our world. So far as we can judge this has 

 always been true. The physical world has undergone great changes. Pe- 

 riods of glacial cold have alternated with periods of tropic heat. The floors 

 of shallow seas have been elevated to form lofty mountain ranges, and the 

 latter have in turn been worn down to low hills and plains, and perhaps 

 eventually covered by the sea once more. Aquatic environments, the home 

 of great proportions of the animal kingdom, have undergone continual 

 change. The oceans have changed the least, yet even here changes have 

 occurred, as, for example, in temperature and in salinity. The oceans have 

 also fluctuated greatly in depth, particularly along the margins of conti- 

 nents. Elevation and subsidence of areas of the earth's crust have been 

 involved in this fluctuation as has, during glacial periods, the locking up of 

 vast quantities of the earth's water supply in polar icecaps and their ex- 

 tensions equator-ward. The environments of fresh-water animals have 

 been even more subject to change. Rivers and lakes are notably short- 



