330 Cell as Basis oj Organic Activity 



Also they can counter by pointing out that no one has seen a "hope- 

 ful monster" develop into a new phylum. They say that the gaps in 

 the geological record are largely due to our incomplete knowledge and 

 the poor preservation of most forms. Answering the criticism that any 

 major change involves many individual characters is likewise simple. 

 Changes occur which have little value in themselves, but which later 

 accumulate to make possible a new mode of life. The reptiles thus show 

 many characters such as a metanephric kidney and the beginnings of a 

 four-chambered heart which are not essential to them but are abso- 

 lutely essential to their homoiothermic descendants, the birds and mam- 

 mals. Such a condition is known as preadaptation, and many examples 

 can be cited. 



The true explanation of the mechanism of evolution still remains to 

 be discovered. Perhaps the truth lies midway between these opposing 

 schools of thought. Certainly in all our thinking it must be realized 

 that there has been time enough to make all things possible. 



ORTHOGENESIS AND THE HISTORY OF THE HORSE 



During the latter part of the nineteenth century, a number of im- 

 portant evolutionary generalizations were formulated. Some few of 

 these "laws" have withstood the test of additional evidence, while others 

 proved useful only to stimulate further work. One of these latter prin- 

 ciples was that known as orthogenesis. This principle stated that many 

 characters developed not at random but in a direct line of progress. Im- 

 pressed by meager fossil evidence, some biologists believed that develop- 

 ment kept on a predestined course, to its ultimate conclusion. This, 

 they reasoned, must be controlled by something within the organism it- 

 self which determined the direction. Evidence which was believed 

 conclusive was the development of the horse's hoof, the development 

 of antlers as shown by the Irish elk, and the excessive development of 

 the canine teeth of the saber-toothed tiger. These latter two animals, 

 particularly, were believed to illustrate the ultimate in straight-line evo- 

 lution. Each developed a particular character to the extreme, and then 

 supposedly this character was so overdeveloped that the animal was 

 doomed to extinction. More probable is that their extinction was due to 

 changes in climate or competition from more efficient animals. 



While the development of the horse seemed at first to be the 

 most convincing evidence for orthogenesis, later study revealed that 

 the horse presented some of the best evidence against such a theor}'. 

 At every stage of development, there were many side lines showing 



