152 THE POPULAR SCIENCE MONTHLY. 



has actually taken place in many cases, and that, in nearly all cases in 

 which the sexes differ in peculiarities not actually concerned in repro- 

 duction, the male has varied more than the female. The amount of 

 variation which any organism has lately undergone may be learned in 

 two ways by a comparison of allied species, and by a comparison of 

 the adult with the young. In a genus which comprises several species 

 the characteristics which these species have in common are due to he- 

 redity from a common ancestor, and are therefore older than features 

 which are confined to any one species. Now, it is a well-known 

 ornithological law that the females of allied species of birds are very 

 much more alike than the males, and that in some cases where the 

 females can hardly be distinguished the males are very conspicuously 

 different so much so that there is not the least danger of confound- 

 ing them. Countless examples will present themselves to anyone who 

 is at all familiar with birds, and those who are not can at once find 

 ample proof by glancing through any illustrated work on ornithology 

 Gould's " Humming-Birds," for example. 



The greater variability of the male is also shown by a comparison 

 of the adult male and female with the immature birds of both sexes. 

 Since the growing animal tends to recapitulate, during its own devel- 

 opment, the changes through which its ancestors have passed, substan- 

 tially in the order in which they first appeared, it follows that, in cases 

 where the sexes are unlike, the one which is most different from the 

 young is the one which has varied. Now, it is only necessary to com- 

 pare the nearly full-grown young of our domestic fowls with the adult 

 cock and hen, to perceive that the adult hen agrees with the young of 

 both sexes in lacking such male characteristics as the highly ornament- 

 ed tail-feathers, the briliant plumage, the distended comb, the spurs, 

 and the capacity to crow. Countless similar illustrations might be 

 given to show the great tendency of the male to vary, but the above 

 are sufficient for the purposes of our argument. As both sexes usually 

 retain the more general specific and generic characteristics, and are 

 alike as far as these are concerned, it is a little more difficult to show 

 the conservative constitution of the female than it is to prove the male 

 tendency to vary. Among the Barnacles there are a few species the 

 males and females of which differ remarkably. The female is an 

 ordinary barnacle, with all the peculiarities of the group fully devel- 

 oped, while the male is a small parasite upon the body of the female, 

 and is so different from the female of its own species, and from all 

 ordinary barnacles, that no one would ever recognize, in the adult 

 male, any affinity whatever to its closest allies. All of the hereditary 

 race characteristics are wanting : the limbs, digestive organs, and 

 most of the muscles and nerves have disappeared, as they are not 

 needed by a parasitic animal ; and the male is little more than a re- 

 productive organ attached to the body of the female. It is only when 

 the development of the male is studied that we obtain any proof of its 



