62 Evolution and Adaptation 
tion, the interpretation that he gave to the recapitulation 
theory did not have the importance that’ it was destined. 
to have when the animals that lived in the past came to 
be looked upon as the ancestors of ,existing animals.!_ But 
with the acceptation of the theory of evolution, which was 
largely the outcome of the publication of Darwin’s “ Origin 
of Species” in 1859, this new interpretation immediately 
blossomed forth. In fact, it became almost a part of the 
new theory to believe that the embryo of higher forms 
recapitulated the series of ancestral adult forms through 
which the species had passed. The one addition of any 
importance to the theory that was added by the Darwinian 
school was that the history of the past, as exemplified by 
the embryonic development, is often falsified. 
Let us return once more to the facts and see which of 
them are regarded at present as demanding an explanation. 
These facts are not very numerous and yet sufficiently ap- 
parent to attract attention at once when known. 
The most interesting case, and the one that has most often 
attracted attention, is the occurrence of gill-clefts in the 
embryos of reptiles, birds, and mammals. These appear 
on each side of the neck in the very early embryo. Each 
is formed by a vertical pouch, that grows out from the wall 
of the pharynx until it meets the skin, and, fusing with the 
latter, the walls of the pouch separate, and a cleft is formed. 
This vertical cleft, placing the cavity of the pharynx in com- 
munication with the outside, is the gill-slit. Similar openings 
in adult fishes put the pharynx in communication with the 
exterior, so that water taken through the mouth passes out 
at the sides of the neck between the gill filaments that border 
the gill-slits. In this way the blood is aerated. The number 
of gill-slits that are found in the embryos of different groups 
1 Carl Vogt in 1842 suggested that fossil species, in their historical succession, 
pass through changes similar to those which the embryos of living forms 
undergo. 
