2 8 RETROGRESSIVE DEVELOPMENT IN NATURE. [IX. 



gressive development can only take place very gradually: it 

 must require many generations to completely eliminate what 

 is superfluous, and we should expect to find in many animals 

 vestiges of organs and structures once significant, but now on 

 the road to complete obliteration. And this is actually the case, 

 as I have shown above. So-called ' rudimentary ' organs are 

 present in numberless cases, and in various animals, and give 

 us some idea of the vast amount of change which every species 

 must have undergone in the course of ages. Of such a kind are 

 degenerate ej^es, hidden beneath the skin, as in the Proteus, 

 the golden mole, and the Caecilia ; the rudimentary wings of the 

 Kiwi, and of many female moths the males of which have well- 

 developed wings ; the almost invisible projections near the 

 mouth of the Ephemendae, which are nothing less than degene- 

 rate jaws ; and a thousand other examples. To the same causes 

 are due the numerous cases in which an organ, fully developed 

 in the ancestors, is wanting in the adult descendant, although 

 present in a rudimentary condition during youth or embryonic 

 life. Thus, the workers of ants are, as before mentioned, wing- 

 less, but the vestiges of wings are still to be seen in the larvae, 

 in the form of small disc-like objects beneath the skin, which 

 subsequently disappear. Thus, too, the larvae of bees have 

 lost their legs, because they do not need to crawl about, but live 

 enclosed in a waxen cell in close proximity to their food : 

 although disuse has thus brought them to the condition of foot- 

 less grubs, in the egg they nevertheless still exhibit vestiges of 

 the legs which their saw-fly-like ancestors must have possessed. 

 Examples like these show that retrogression in an organ, which 

 degenerates from disuse, takes place first in the mature stage, and 

 does not extend to the embryonic stages until much later. An 

 organ may persist in the embryo for thousands of generations 

 after it has been ehminated from the adult organization. The 

 history of evolution affords many wefl-authenticated instances 

 of organs which persist in a rudimentary condition and never 

 attain a higher development. They are, of course, of the 

 greatest importance as throwing light upon the past history of 

 a species, and are in themselves sufficient proof of the number 

 and diversity of the ancestors of existing species ; they show 

 us how intricate and devious are the workings of nature in the 

 evolution of the organic world— now progressive, now retro- 



