ix EMBEYOLOGY AND EVOLUTION 503 



become known from a series of geological deposits A, B, G, D, this 

 affords convincing evidence in most cases that the particular 

 organisms lived at the time the particular deposits were laid down : 

 the conclusion may also be fairly justifiable that not only did they 

 exist but that they were abundant at the period in question. The 

 conclusion however which is so apt to be drawn that a, b,c, d, actu- 

 ally made their first appearance in the same order as the deposits 

 A, B, C, B, is quite unreliable. They may have existed in smaller 

 numbers for immense periods of time before the periods corresponding 

 to A, B, C, B, when they were really abundant, and the order of their 

 first appearance may have been d, c, I, a, or any other. Such a 

 geological series is in fact in itself of little value as an index to the 

 order of evolution. In Embryology on the other hand where the 

 evolutionary stages occur as part of a continuous process, each 

 dependent upon its predecessor, we appear to be safe in assuming 

 that the record, however incomplete, is at least arranged in proper 

 sequence. 



Another principle to be borne in mind, when the attempt is being 

 made to work out the evolutionary, history of a particular group, is 

 that conclusions must be based upon broad knowledge of structure as 

 a whole. No implicit reliance must be placed upon evidence relating 

 to one system of organs unless it is corroborated by the evidence of 

 other organs. Failing this precaution the investigator is liable to 

 the pitfall afforded by convergent evolution of organs of similar 

 function. Here again the palaeontologist finds himself much hampered 

 as compared with the embryologist, for as a rule all evidence except 

 that of the skeletal system has passed completely beyond his ken. 



Evolutionary Origin of the Vertebrata.' — In the preceding 

 portions of this book it has been shown that Embryology provides us 

 with a record — in at least its main outlines — of the evolutionary 

 changes which the various organ-systems have undergone within the 

 group Vertebrata. For, amongst others, the reasons stated at the 

 foot of p. 491 the record is less clear regarding the evolutionary 

 history of the complete individual. Even however if we had this 

 record complete for the various types — Fish, Amphibian, Eeptile, 

 Bird — W e should find ourselves still confronted with the interesting 

 problem of the first origin of the primitive Vertebrate type : — from 

 whence came these lowly original Vertebrates out of which the various 

 existing types of Vertebrate have been evolved ? 



This problem of the ancestry of the Vertebrata is naturally a 

 fascinating one and it has attracted much attention and been the 

 theme of voluminous writing. Enthusiasts have at different times 

 endeavoured to demonstrate that the Vertebrates are descended from 

 this phylum or from that. It is perhaps best not to take such 

 attempts very seriously. They have served a useful purpose in 

 arousing interest and stimulating research but they have little claim 

 to a place in the permanent literature of Zoology. . 



We are naturally unable to get any evidence bearing upon the 



