BIRDS AND EVOLUTION 365 



(c) Historical. — Not much can be made of the fact that 

 some extinct reptiles could in some measure fly ; there is 

 more importance in the fact that some of them were bipeds. 

 The reason why no stress is laid on the flying Pterodactyls is 

 that their wing was of a quite different type from the bird's 

 wing. There is satisfactory historical evidence, however, 

 in the oldest known bird, Archasopteryx, for it had several 

 reptilian features, such as teeth, a long tail, a half-made wing 

 with claws on three digits, and abdominal ribs. There are 

 some other reptilian features in some other extinct birds 

 which are not so ancient as Archasopteryx. 



Let us take another little point — a straw that shows how 

 the wind has blown. Duerden (19 19) points out that at 

 a certain stage in its development the two-toed ostrich 

 (Struthio) has three toes and hints of another or of other 

 two. That is to say, two or three toes have been lost, but 

 the loss is not quite complete. This embryonic persistence 

 is unintelligible except on the evolutionary interpretation 

 that the two-toed ostrich is descended from ancestors with 

 four or five. 



§ 2. Factors in Evolution 



Accepting the fact of evolution, what can we say in regard 

 to the factors that may have operated in bringing it about .'' 

 Our knowledge of these factors remains very imperfect, and 

 we are in great part restricted to an argument from analogy. 

 We are forced in the main to argue from what goes on in the 

 present to what may have gone on in the dim and distant 

 past. There may be factors in evolution which have not yet 

 been discovered, and there are possible factors {e.g. the 

 transmission of acquired characters or bodily modifications) 

 in regard to which we are in doubt. It seems useful to 

 distinguish the originative factors which bring about new 

 departures or variations, and the directive factors which sift 

 or prune this material. Besides these two sets of factors 

 there is heredity or the relation between successive genera- 

 tions, which determines whether a novelty will or will not be 

 entailed on the next generation. Let us begin with the 

 originative factors. 



