FINAL REMARKS 497 
with the result that, in the minds of many, embryology has failed to 
afford the desired clue. 
At the same time, the geological record was looked upon as too 
imperfect to afford any real help; it was said, and is said, that the 
Cambrian and pre-Cambrian periods were so immense, and the animals 
discovered in the lower Silurian so highly organized, as to compel 
us to ascribe the origination of all the present-day groups to this 
immense early period, the animals of which have left no trace of 
their existence as fossils. 
In consequence of, or at all events following upon, the supposed 
failure of embryology and of geology to solve the problem of the 
sequence of evolution of animal life, a new theory has arisen, which 
goes very near to the denial of evolution altogether. This is the 
theory of parallel development. It discards the old picture of a genealo- 
gical tree with main branches arising at different heights, these again 
branching and branching into smaller and smaller twigs,and substitutes 
instead the picture of the ribs of a fan, every rib running independently 
of every other, each group represented by a rib reaching its highest 
development on the circumference of the fan and coming nearer 
and nearer to a common point at the handle of the fan. This point 
of convergence, where all the groups ultimately meet, is so far back 
as to reach to the lowest living organisms. 
This, in my opinion, unscientific and inconceivable suggestion has 
arisen largely in consequence of a conception which has become 
firmly fixed in the minds of very many writers on this subject—the 
conception that in the evolution of every group, the higher members 
of the group are the most specialized in the peculiarities of that group, 
and it is impossible to obtain a new group with different peculiarities 
from such specialized members. If, then, a higher group is to arise 
from a lower, it must arise from the generalized members of that 
lower group, in other words, from the lowest members or those 
nearly akin to the next lower group. 
Similarly, the highest members of this latter group are too 
specialized, and again we must go to the more generalized members 
of the group. In this way each separate specialized group is put on 
one side, and so the conception of parallel development comes into 
being. 
The evidence given in this book dealing with the origin of 
vertebrates strikes at the foundations of this belief, for it presents an 
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