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. 1 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. 



