RUDIMENTARY ORGANS. 35 



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 re- 

 mains as the sole axial skeleton in Aviphioxus, likewise 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 back- 

 bone, seeing that the former does not become the latter ? 

 Kleinenberg's suggestion is that the notochord supplies the 

 stimulus, the necessary condition, 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. 



Rudimentary Organs. 



In many animals there are structures which attain no 

 complete development, which are rudimentary in com- 

 parison 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 incom- 

 pletely formed. Other organs may be similarly spoilt in 

 the making. They illustrate arrested development. (p) 

 Some animals lose, in the course of their life, some of the 

 promiseful characteristics of their larval life ; thus parasitic 

 crustaceans at first free-living, and sessile sea squirts at 

 first free-swimming, always undergo degeneration. The 

 retrogression can be seen in each lifetime. But the little 

 Kiwi of New Zealand, with mere apologies for wings, and 

 many cave fishes and cave crustaceans with slight hints of 

 eyes, illustrate degeneration which has taken such a hold of 

 the animals that the young stages also are degenerate. The 

 retrogression cannot be seen in each lifetime, evident as it 



