456 THE ORIGIN OF VERTEBRATES 



absolutely impossible that it only requires to be stated to be dis- 

 missed as au absurdity. 



Against this opinion I claim boldly that my theory is not only 

 not contrary to the principles of embryology, but is mainly based 

 upon the teachings of embryology. I wish here not to be mis- 

 understood. The great value of the study of embryology for questions 

 of the sequence of the evolution of animals is to be found in what is 

 known as the Law of Becapitulation, which asserts that every animal 

 gives some indication in the stages of its individual development of 

 its ancestral history. Naturally enough it cannot pass through all 

 the stages of its past history with equal clearness, for what has taken 

 millions of years to be evolved has to be compressed into an evolution 

 lasting only a few months or weeks, or even less. 



When in the highest vertebrate a vestigial organ, such as the 

 pineal gland, can be traced back without leaving the vertebrate 

 kingdom to a distinct median eye, such as is found in the lamprey, 

 that rudimentary organ is evidence of an organ which was functional 

 in the earliest vertebrates or their immediate ancestors. So it is 

 generally with well denned vestigial organs found in the adult 

 animal ; they always indicate an organ which was functional in the 

 near ancestor. 



Passing from the adult to the embryo we still find the same law. 

 Here, also, vestigial organs are met with, which may leave no trace in 

 the adult, but indicate organs which were functional in the near 

 ancestor. Thus, but for embryology, we should have no certainty 

 that the air-breathing vertebrates had been derived from water- 

 breathing fishes ; the indication is not given by any close resemblance 

 between the formation of the embryos in their earliest stages, but 

 by the formation of vestigial gill-arches even in the embryos of the 

 highest mammal. 



For all questions of evolution the presence of vestigial organs in 

 the embryo is the important consideration, for they give an indication 

 of near ancestry ; the early formation of the embryo concerns a 

 much more remote ancestral period, all vestigial organs of which 

 may well have been lost and obscured by coenogenetic changes. Let 

 us, then, consider the two things — the vestigial organs and the early 

 formation of the embryo — separately, and see how far my opponents 

 are justified in their statement that my theory contravenes the 

 principles of embryology. 



