THE EVOLUTIONARY CONCEPT 337 



about 2 billion 500 million years ago, and that as early as 1 billion 800 

 million years ago it had reached something like its present physical 

 state and may have been capable of supporting life. Actual traces of 

 life have been found in rocks that, by the best methods of estimate, are 

 between 600 and 900 million years old. One need not take these figures 

 too literally ; they are chiefly of interest as indicative of the general order 

 of time magnitudes involved. Probably no one would be seriously per- 

 turbed if it were found necessary to cut the estimates in half. From the 

 standpoint of the biologist it would seem of little moment whether life 

 has endured for 1,800 million, 1,000 million, or even a mere 500 million 

 years; any of these times seems amply long to have permitted the occur- 

 rence of the changes in life that are recorded in the rocks. 



Attempts to formulate an evolutionary hypothesis. Although no 

 satisfactory theory of evolution could be formulated prior to the establish- 

 ment of the uniformitarian principle and recognition of the great age of 

 the earth, one should not suppose that evolutionary speculation remained 

 at a standstill until after the appearance of Hutton's work. All through 

 the eighteenth century, developments in many branches of science made 

 it increasingly evident that the doctrine of special creation was inade- 

 quate. As geological exploration widened, the abrupt breaks between 

 the faunas of contiguous rock formations that were supposed to have been 

 caused by catastrophic extinctions followed by new creations, were in 

 many instances found to be bridged over in the rocks of other regions. In 

 numerous groups fossils were seen to show a gradual transition from 

 types quite unlike modern ones, in the older rocks, to other types increas- 

 ingly like those of today in the more recent strata. Exploration of distant 

 regions brought new knowledge of their faunas and floras. The tremendous 

 number and variety of kinds of organisms came to be more fully appre- 

 ciated, and it eventually became certain that most of the fossil types were 

 no longer living anywhere on earth. Comparative anatomical studies 

 were revealing a multitude of concealed but fundamental resemblances 

 between superficially unlike animals and plants. Unexplained likenesses 

 were being found between the early embryonic stages of animals that 

 differed greatly as adults. The doctrine of special creation offered no 

 satisfactory explanation of these and great numbers of other new facts, 

 which were to fall so neatly into place and acquire so clear a significance 

 under the theory of evolution. 



More and more persons came to feel that these phenomena must have 

 some meaning, if it could only be grasped; and by the latter part of the 

 eighteenth and the first part of the nineteenth centuries a number of 

 biologists and naturalist-philosophers were seeking some consistent and 

 adequate theory that would account for them. The earlier of these men 

 were merely feeling their way, their ideas being very vague, incomplete, 



