ORGANS, 39 
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 
similarly spoilt in the making. They illustrate arrested 
development. (6) 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. (d) But among “rudimentary organs ” 
we also include structures somewhat different, e.g. the gill- 
