XXI.] EMBKYOLOGY. 259 



besides the red corpuscles there are white ones, whicli are Blood- 

 similar to those found in Invertebrates. In the develop- ^X'trand' 

 ment of the vertebrate embryo the white corpuscles appear red : 

 earlier than the red ; and it is now believed that through- 

 out life the red corpuscles continue to be formed by the 

 metamorphosis of the white. So that we have this re- 

 markable threefold relation between the two kinds : that their 

 the white are found in lower oro-anic forms than the red ; threefold 

 the white appear in the embryo before the red ; and the 

 white are transformed into the red throughout life.^ 



What inference are we to draw from these facts of in- 

 direct development ? and especially from the resemblance 

 which we find to be so general, in various ways, through- 

 out the animal kingdom, between the embryo or larva of 

 the higher form, and the mature state of the kindred lower 

 form ? How are we to interpret the facts, that the lower 

 aquatic Invertebrata usually begin their life in the likeness 

 of Protozoa ; that the branchiae of the higher Crustacea are 

 at first similar to those of the lower Crustacea ; that many 

 insects leave the egg as worms ; that the frog and newt 

 have the respiratory system of fishes, and consequently 

 their circulatory system also, before they acquire those of 

 air-breathers ; and, most surprising of all, that the embryos 

 of the higher air-breathing Vertebrata at first develop 

 " branchial slits " and a branchial circulation hke those of 

 fishes, with the arteries dividing as the arteries of fishes 

 divide ? 



It is a good rule, in all questions of this kind, to begin Cau these 

 by trying whether the facts are capable of being all referred referred to 

 to Cuvier's principle of the adaptation of every organ and the prin- 

 every function in an organism to the rest, and of all to its adapta- 

 mode of life. But if it is difficult to apply this principle *^°" • 

 with any precision to the morphology of organisms, it is 

 still more difficult to apply it to their development ; if it 

 is difficult to be sure that we thoroughly understand the 

 purpose of any peculiarity of form or structure in the 

 organism considered simply as a living mechanism, it is 

 quite impossible to be sure that we understand all the 



1 Carpenter's Human Physiology, p. 163. 



s2 



