68 THE POPULAR SCIENCE MONTHLY. 



further paleontology has progressed, the more it has tended to fill up 

 the gaps between existing groups and species ; while the careful study 

 of living forms has brought into prominence the variations dependent 

 on food, climate, habitat, and other conditions, and shown that many 

 species, long supposed to be absolutely distinct, are so closely linked 

 together by intermediate forms that it is difficult to draw a satisfac- 

 tory line between them. 



The principles of classification point also in the same direction, and 

 are based more and more on the theory of descent. Biologists en- 

 deavor to arrange animals on what is called the "natural system." No 

 one now places whales among fish, bats among birds, or shrews with 

 mice, notwithstanding their external similarity ; and Darwin main- 

 tained that " community of descent was the hidden bond which natu- 

 ralists had been unconsciously seeking." How else, indeed, can we 

 explain the fact that the framework of bones is so similar in the arm 

 of a man, the wing of a bat, the foreleg of a horse, and the fin of a 

 porpoise that the neck of a giraffe and that of an elephant contain the 

 same number of vertebrae ? 



Strong evidence is, moreover, afforded by embryology ; by the 

 presence of rudimentary organs and transient characters, as, for in- 

 stance, the existence in the calf of certain teeth which never cut the 

 gums, the shriveled and useless wings of some beetles, the presence of 

 a series of arteries in the embryos of the higher vertebrata exactly 

 similar to those which supply the gills in fishes, even the spots on the 

 young blackbird, the stripes on the lion's cub ; these, and innumerable 

 other facts of the same character, appear to be incompatible with the 

 idea that each species was specially and independently created ; and 

 to prove, on the contrary, that the embryonic stages of species show us 

 more or less clearly the structure of their ancestors. 



Darwin's views, however, are still much misunderstood. I believe 

 there are thousands who consider that according to his theory a sheep 

 might turn into a cow, or a zebra into a horse. No one would more 

 confidently withstand any such hypothesis, his view being, of course, 

 not that the one could be changed into the other, but that both are 

 descended from a common ancestor. No one, at any rate, will question 

 the immense impulse which Darwin has given to the study of natural 

 history, the number of new views he has opened up, and the additional 

 interest which he has aroused in, and contributed to, biology. "When 

 we were young, we knew that the leopard had spots, the tiger was 

 striped, and the lion tawny ; but why this was so it did not occur to 

 us to ask ; [and, if we had asked], no one would have answered. Now 

 we see at a glance that the stripes of the tiger have reference to its life 

 among jungle-grasses ; the lion is sandy, like the desert ; while the 

 markings of the leopard resemble spots of sunshine glancing through 

 the leaves. 



The science of embryology may almost be said to have been ere- 



