90 ADAM SEDGWICK. 



must not be forgotten that some of them are functional and 

 that these resemble organs of larvae in retaining ancestral 

 featureSj e. g. the ductus arteriosus, &c.] 



To put the matter in another and more general way, the only 

 functionless ancestral structures which are preserved in develop- 

 ment are those which at some time or another have been of 

 use to the organism during its development after they have 

 ceased to be so in the. adult. In this way I should be inclined 

 to explain the hair of the human foetus and the teeth of the 

 foetal whale — that is to say, I should be inclined to suppose that 

 the possession of the lanugo is due to the fact that there was a 

 time in the evolution of man when the babe required this 

 protection against the cold after the necessity for it had dis- 

 appeared in the adult, and that the young whale in the days 

 when whalebone was first acquired still retained the ancestral 

 habits which required teeth. It is, however, possible that 

 these and other similar cases of the retention of rudimentary 

 organs in late embryonic life have another explanation, and^it 

 becomes necessary to collect and examine as many cases as 

 possible of the undoubted retention, as embryonic rudiments, of 

 organs which we have reason to know have recently disappeared 

 from the adult stage. 



The retention of such organs in the embryo may, as I have 

 hinted, be due to the fact that they have been retained func- 

 tionally by the young animal after they have been lost by the 

 adult; but another explanation is possible, which is that 

 organs which are becoming functionless, and disappearing at 

 all stages, may in some cases disappear unevenly ; that is to 

 say, they may remain at one stage after they have totally dis- 

 appeared at another. In this manner we might get an organ 

 which had become quite functionless and had quite disap- 

 peared in the free stage, still persisting, though with a much 

 reduced development, in the embryo. It is possible that the 

 lanugo and the teeth of foetal whales may be explicable in this 

 manner. But that such a retention of organs in the embryo 

 is not an important or permanent one is shown by the fact of 

 their comparative scarcity in embryonic histories. This is a 



