108 GENERAL CONSIDERATIONS. 



and the form of the ancestral Protozoon has often been 

 criticised, and it seems to me that the facts suggest, with 

 equal strength, quite another view of the matter, viz. that 

 which I have just hinted at. 



There are several ways suggested by embryology in which 

 the passage from the Protozoa to the Metazoa may have 

 been effected ; and a most admirable and profound analysis of 

 each of these, and a critical review of our knowledge on this 

 subject, is to be found in chap, xiii, vol. ii, of the ' Comparative 

 Embryology.' I cannot do better than quote the words in 

 which Balfour sums up this review of the facts — "Considering 

 the almost indisputable fact that both the processes above 

 dealt with [delamination and invagination] have in many 

 instances had a purely secondary origin, no valid arguments 

 can be produced to show that either of them reproduces the 

 mode of passage between the Protozoa and the ancestral two- 

 layered Metazoa. These conclusions do not, however, throw 

 any doubt upon the fact that the gastrula, however evolved, 

 was a primitive form of the Metazoa; since this 

 conclusion is founded upon the actual existence of 

 adult gastrula forms independently of their occur- 

 rence in development" (' Comp. Emb.,' vol. ii, p. 283; the 

 italics are mine). 



These words seem to me to express as clearly now, as they 

 did when they were written five years ago, the state of our 

 knowledge on this subject, and in my opinion neither Metsch- 

 nikoff's view, nor that which I have just put forward as to the 

 exact method of transition between the Protozoa and Metazoa, 

 can be regarded as anything more than more or less plausible 

 suggestions without any strong basis of fact. 



The gastrsea theory, in so far as it implies the existence of an 

 ancestral two-layered organism, is still in accordance with 

 known facts, and no discoveries have been made which deci- 

 sively settle the mode of transition between the Protozoa and 

 Metazoa. As, however, the subject is an interesting one it 

 seems worth while contrasting Metschnikoff's view with that 

 which I have just put forward. But before doing so, I am anxious 



