456 THE ORIGIN OF VERTEBRATES 
absolutely impossible that it only requires to be stated to be dis- 
missed as an 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 Recapitulation, 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 defined 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 cosnogenetic 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. 
