ORGANS. 39 



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 

 similarly spoilt in the making. They illustrate arrested 

 development. (l>) Some animals lose, in the course of their 

 life, many of the prominent characteristics of their larval 

 life ; thus parasitic crustaceans at first free-living, and sessile 

 sea-squirts at first free-swimming, always undergo degenera- 

 tion, which 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 degener- 

 ate. The retrogression cannot be seen in each lifetime, 

 evident as it is when we compare these degenerate forms with 

 probable ancestors, (c) But among " rudimentary organs " 

 we also include structures somewhat different, e.g. the gill- 

 clefts which persist in embryonic reptiles, birds, and 



