20 INVERTEBEATA CHAP. 



on as stationary in comparison, and spoken of as larval stages or 

 instars. 



Now a great deal of the interest in the science of embryology has 

 arisen from the fact that both the embryonic and larval phases of 

 development show features which have been interpreted as being a 

 reproduction of the characters of far-off ancestors of the species to 

 which the adult belongs. This theory is the so-called fundamental 

 law of biogenetics, and is summed up in the phrase, " The individual 

 in its development recapitulates the development of the race." If 

 this " law " can be substantiated the interest in embryology becomes 

 immense, it binds all the innumerable phenomena of development 

 into one coherent scheme, and opens the door to the hope that we 

 may yet be able to sketch the main history of life on the earth. 



The direct evidence of this history, as contained in the fossil record, 

 takes us back only a short distance. In the lower Cambrian rocks 

 the great groups of Mollusca, Arthropoda, Echinodermata, and 

 Brachiopoda are all as sharply marked off from one another as at 

 the present day, and since only hard parts are preserved, the all- 

 important " soft " parts which constitute the real living matter are 

 irrecoverably lost, and no trace is left of an organism except it 

 possessed some kind of skeleton. But an egg has many points of 

 resemblance to the simplest animals, the Protozoa, and if development 

 be really a recapitulation of ancestral history, then the whole of the 

 ancestral history of an animal, from the Protozoan stage to the 

 present, should be presented in outline in its Life-history. 



But although most naturalists would agree that a life-history 

 contains ancestral " elements," all would be emphatic on the subject 

 that it likewise exhibits many features which are purely secondary, 

 and in no way reflect the characters of ancestors. With these general 

 considerations the agreement stops. As the founder of the Naples 

 Biological Station has caustically remarked, it is a curious fact that 

 every investigator is convinced tbat the type which he is studying has 

 a monopoly of most of the primitive features, and that other types are 

 secondarily modified. The endless wrangles about primitive and 

 secondary features, which have made up so much of embryological 

 writing for the past forty years, have so disgusted many leading 

 workers in this field, that they have been inclined to go in the 

 opposite extreme, and deny altogether the "biogenetic law." Driesch 

 may be mentioned as an example of this, and a very searching 

 criticism of the whole hypothesis is given in the Darwin memorial 

 volume by Sedgwick (1909), who in former years had done more than 

 most workers to illuminate the hypothesis. 



Driesch's (1907 and 1908) criticism leads him to the position that 

 the development of an egg into an adult is not to be explained by 

 physical and chemical laws, and he therefore attributes to each 

 species of animal a peculiar " entelechy " or soul, which presides over 

 the task of making its germs develop. Thus we are brought back to 

 pre-Darwinian days, to a position indeed more primitive than that 



