4 THE EVOLUTION THEORY 



This prediction, with another to be explained later, is based upon 

 the experience that the development of an individual animal follows, 

 in a general way, the same course as the racial evolution of the 

 species, so that structures of the ancestors of a species, even if they 

 are not found in the fully developed animal, may occur in one of its 

 earlier embryonic stages. Further on. we shall come to know this 

 fact more intimately as a 'biogenetic law,' and it alone would be 

 almost enough to justify the theory of evolution. Thus, for instance, 

 the lowest vertebrates, the Fishes, breathe by means of gills, and these 

 breathing organs are supported by four or more gill-arches, between 

 which spaces, the gill-slits, remain open for the passage of water. 

 Although Reptiles, Birds, and Mammals breathe by lungs, and at no 

 time of their life by gills, yet, in their earliest youth, that is, during 

 their early development in the egg, they possess these gill-arches and 

 gill-slits, which subsequently disappear, or are transformed into other 

 structures. 



On the strength of this ' biogenetic law ' it could also be predicted 

 that Man, in whom, as is well known, there are twelve pairs of ribs, 

 would, in his earliest youth, possess a thirteenth pair, for the lower 

 Mammals have more numerous ribs, and even our nearest relatives, 

 the anthropoid Apes, the gorilla and chimpanzee, have a thirteenth 

 rib, though a very small one, and the siamang has even a fourteenth. 

 This prediction also has been verified by the examination of young 

 human embryos, in which a small thirteenth rib is present, though it 

 rapidly disappears. 



During the seventies I was engaged in investigating the develop- 

 ment of the curious marking which adorns the long body of many 

 of our caterpillars. I studied in particular the caterpillars of our 

 Sphingida3 or hawk-moths, and found, b}^ a comparison of the various 

 stages of development from the emergence of the caterpillar from the 

 egg on to its full growth, that there is a definite succession of 

 different kinds of markings following each other, in a whole range 

 of species, in a similar manner. From the standpoint of the 

 Evolution theory, I concluded that the markings of the youngest 

 caterpillars, simple longitudinal stripes, must have been those of the 

 most remote ancestors of our present species, while those of the later 

 stages, oblique stripes, were those of ancestors of a later date. 



If this were the case, then all the species of caterpillar which 

 now exhibit oblique stripes in their full-grown stage must have had 

 longitudinal stripes in their youngest stages, and because of this 

 succession of markings in the individual development, I was able to 

 predict that the then unknown young form of the caterpillar of our 



