SUBSTITUTION OF ORGANS 45 



the placenta which effects vital connection between off- 

 spring and mother. 



Substitution of organs. — The idea of several changes of 

 function in the evolution of an organ, suggests another of 

 not less importance which has been emphasised by Kleinen- 

 berg. An illustration will explain it. In the early stages 

 of all vertebrate embryos, the supporting axial skeleton is 

 the notochord — a rod developed along the dorsal wall of 

 the gut. From Fishes onwards, this embryonic axis is 

 gradually replaced in development by the vertebral column 

 or backbone ; the notochord does not become the back- 

 bone, but is replaced by it. It is a temporary structure, 

 around which the vertebral column is constructed, as a tall 

 chimney may be built around an internal scaffolding of 

 wood. Yet it remains as the sole axial skeleton in 

 Amphioxus, persists in great part in hag and lamprey, but 

 becomes less and less persistent in Fishes and higher 

 Vertebrates, as its substitute, the backbone, develops more 

 perfectly. Now, what is the relation between the notochord 

 and its substitute the backbone, seeing that the former does 

 not become the latter ? Kleinenberg's suggestion is that 

 the notochord supplies the stimulus, the necessary condi- 

 tion, for the formation of the backbone. Of course we 

 require to know more about the way in which an old- 

 fashioned structure may stimulate the growth of its future 

 substitute, but the general idea of one organ leading on to 

 another is suggestive. It is consistent with our general 

 conception of development — that each stage supplies the 

 necessary stimulus for the next step ; it also helps us to 

 understand more clearly how new structures, too incipient 

 to be of use, may persist, and why old structures should 

 linger though they have only a transitory importance. 



Rudimentary organs. — In many animals there are struc- 

 tures which attain no complete development, which are 

 rudimentary in comparison with those of related forms, and 

 seem retrogressive when compared with their promise in 

 embryonic life. But it is necessary to distinguish various 

 kinds of rudimentary structures, (a) As a pathological 

 variation, probably due to some germinal defect, or to the 

 insufficient nutrition of the embryo, the heart of a mammal 

 is sometimes incompletely formed, Other organs may be 



