498 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 
