9i6 LIFE : OUTLINES OF GENERAL BIOLOGY 



an extraordinary complexifying (as among sponges and corals) 

 without real advance, and many extinctions of fine types (such as 

 the Pterodactyls or Flying Dragons), there has been on the whole 

 an emergence of progressively more differentiated and more inte- 

 grated forms of life. And organic evolution implies not merely that, 

 say, modem crocodiles arose from more generalised extinct 

 crocodilians ; it implies the emergence of distinct novelties, such as 

 insects from non-flying ancestors, or birds from a terrestrial stock 

 of bipedal reptiles. 



The logical position of the general idea of Organic Evolution is 

 not like that of the law of gravitation or the laws of the conservation 

 of matter and energy, where experimental verification is readily 

 forthcoming. The evolution-idea suggests a genetic description of 

 how organisms have come to be as they are, and its validity rests 

 on the way it fits the facts. It defines a certain mode of becoming', 

 and there are no data which are in any way discrepant or contra- 

 dictory. Although zoologists do not at present know the gradations 

 by which the first known bird (Archseopteryx) arose from an extinct 

 reptile stock (probably the order of Pseudosuchia), no competent 

 zoologist doubts that birds evolved from reptiles. And although 

 there is great difference of opinion among zoologists in regard to the 

 causal factors which operated in the evolution of birds, there is no 

 hesitancy in saying that the evolution-idea is, so far as it goes, a 

 broadly satisfactory formulation of the mode of becoming that has 

 actually taken place. 



An illustration from the study of individual development may be 

 of service. For centuries it was believed that various parasites could 

 arise "spontaneously" within their host, and the larvae of flies 

 within the exposed flesh. The descriptive formula of the mode of 

 becoming was "Spontaneous Generation" or "Abiogenesis". But 

 through the patient work of many naturalists, from Redi to Pasteur, 

 it became quite clear that the parasites and the maggots arose from 

 parents and from eggs in the usual way, though sometimes dis- 

 guisedly. Thus the descriptive formula of their mode of becoming was 

 recognised as the old and familiar term Development. This remains 

 quite convincing even though there are still many cases where the 

 stages in the life-history are inadequately known, and although no 

 embryologist can say more than a little in regard to the factors 

 operative in the developmental process. So is it in regard to racial 

 evolution. 



EVIDENCES OF EVOLUTION.— Every detail of botany and 

 zoology that has been carefully studied admits of evolutionary 

 description, and more than one specialist has offered to vindicate 

 the general idea of Organic Evolution without going beyond 

 the class or group to which he had given particular attention. 



