2o6 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



stages representing in proper chronological order some 

 of the stages of adult structure which have been passed 

 through in the course of evolution." Again: "The 

 early stages of the fish embryo are very like those of 

 the bird embryo. These two do correspond to each 

 other. The statement that the embryonic structure 

 of a bird follows a course which is from beginning to 

 end roughly parallel with, but somewhat divergent 

 from, the course followed by a fish, is borne out by 

 the actual facts. A bird does not develop into a fish 

 and then into a reptile, and then into a bird. There is 

 no fish-stage, no reptile-stage, in its ontogeny. The 

 adult resembles an adult fish only very remotely. Every 

 earlier stage resembles the corresponding earlier stage 

 of the fish more closely. There is a parallelism be- 

 tween the two ontogenies. There is no parallelism be- 

 tween the ontogeny and the phytogeny of either ^ bird or 

 any other animal whatever. A seeming parallelism will 

 fall through when closely examined." " The promise 

 that this theory gave of serving as the guide to knowl- 

 edge of past history without the labor involved in pale- 

 ontological research, was indeed tempting : and where 

 the royal road to learning has been shown by it, it is not 

 surprising that some zoologists should have entered 

 for the race along this road. To what goal that road 

 has led may be learned by a comparison of the nu- 

 merous theories as to the ancestry of the ' Chordata ' 

 which have been put forward by those who have 

 adopted the theory without enquiring as to its valid- 

 ity." 



I have made this quotation as showing the point of 

 view from which the doctrine of parallelism when in- 

 correctly stated may be assailed. There is truth in 

 the author's accusation that embryologists who have 



