EARLIEST FORMS OF HIGHER PHYLA 31 



brian and Silurian, they are flourishing in full force, while 

 their numerous specialized forms are a positive proof of 

 a long antecedent history within the limits of the Phylum. 



If, however, we assume for the moment that the 

 Phyla began in the Cambrian, the geologist's estimate 

 must still be increased considerably, and perhaps 

 doubled, in order to account for the evolution of 

 the higher Phyla from forms as low as many which 

 are now known upon the earth ; unless, indeed, it is 

 supposed, against the weight of all such evidence as 

 is available, that the evolutionary history in these early 

 times was comparatively rapid. 



To recapitulate, if we represent the history of animal 

 evolution by the form of a tree, we find that the follow- 

 ing growth took place in some age antecedent to the 

 earliest fossil records, before the establishment of the 

 higher Phyla of the Animal Kingdom. The main 

 trunk representing the lower Protozoa divided, originating 

 the higher Protozoa ; the latter portion again divided, 

 probably in a threefold manner, originating the two 

 lowest Metazoan Phyla, constituting the Coelentera. 

 The branch representing the higher of these Phyla, 

 the Nematophora, divided, originating the lower Coelo- 

 mate Phyla, which again branched and originated the 

 higher Phyla. And, as has been shown above, the 

 relatively ancestral line, at every stage of this complex 

 history, after originating some higher line, itself con- 

 tinued down to the present day, throughout the whole 

 series of fossiliferous rocks, with but little change in 

 its general characters, and practically nothing in the 

 way of progressive evolution. Evidences of marked 

 advance are to be found alone in the most advanced 

 groups of the latest highest products — the Phyla formed 

 by the last of these divisions. 



It may be asked how is it possible for the zoologist 

 to feel so confident as to the past history of the various 

 animal groups. I have already explained that he does 

 not feel this confidence as reg^ards the details of the 

 history, but as to its general lines. The evidence which 



