49§ THE ORIGIN OF VERTEBRATES 



image of the sequence of evolution of animal forms in orderly upward 

 progress, caused by the struggle for existence among the members of 

 the race dominant at the time, which brought about the origin of the 

 next higher group not from the lowest members of the dominant 

 group, but from some one of the higher members of that group. 



The great factor in evolution has been throughout the growth of 

 the central nervous system ; from that group of animals which 

 possessed the highest nervous system evolved up to that time the 

 next higher group must have arisen. 



In this way we can trace without a break, always following out 

 the same law, the evolution of man from the mammal, the mammal 

 from the reptile, the reptile from the amphibian, the amphibian 

 from the fish, the fish from the arthropod, the arthropod from 

 the annelid, and we may be hopeful that the same law will enable 

 us to arrange in orderly sequence all the groups in the animal 

 kingdom. 



This very same law of the paramount importance of the develop- 

 ment of the central nervous system for all upward progress will, I 

 firmly believe, lead to the establishment of a new and more fruitful 

 embryology, the leading feature of which will be, as suggested in the 

 last chapter, not the attempt to derive from the blastula three germ- 

 layers common to all animals, but rather two sets of organs — those 

 which are governed by the nervous system and those which are not — 

 and thus by means of the development of the central nervous system 

 obtain from embryology surer indications of relationship than are 

 given at present. 



The great law of recapitulation, which asserts that the past 

 history of the race is indicated more or less in the development of 

 each individual, a law which of late years has fallen somewhat into 

 disrepute, owing especially to the difficulty of interpreting the 

 embryological history of the vertebrate, is triumphantly vindicated 

 by the theory put forward in this book. Each separate vertebrate 

 organ, one after the other, as shown in the last chapter, indicates in its 

 development the manner in which it arose from the corresponding- 

 organ of the arthropod. There is no failure in the evidence of 

 embryology, the failure is in the interpretation thereof. 



So, too, my theory vindicates the geological method. There is no 

 failure here ; on the contrary, the record of the rocks proclaims with 

 startling clearness not only the sequence of evolution in the 



