1 82 CHAPTERS ON EVOLUTION. 



the two hindermost folds disappearing in the chick, without leav- 

 ing any traces of their existence. The limbs (Fig. 97, w, /) begin to 

 be developed about the fourth day, and first appear as little buds 

 projecting from a ridge the " Wolffian ridge " running round the 

 young being from neck to tail at about its middle portion ; but it 

 is only about the fifth day that the distinctive characters of the limbs 

 can be discerned. By the tenth day, however, the wings and feet, in 

 all their characteristic structure, may be distinguished; The skull 

 dates its history from the fifth day ; and only during the succeeding 

 day may the bird-type of the chick be perceived in the characters of 

 wings, feet, digestive system, and other stnictures so remarkably 

 alike are the developing young of higher Vertebrates in their earlier 

 stages of development. Meanwhile as early, indeed, as the third 

 day of life the lungs have been formed as little pocket-like growths 

 from the throat; and even before hatching, the chick begins to use its 

 breathing organs-. With fully formed parts, and perfectly equipped 

 for the new existence which lies before it, the chick duly breaks the 

 shell with its armed beak, and, throwing off the shrivelled remnants of 

 organs once useful in- its earlier stages, enters upon the characteristic 

 life of its species. 



If the development of a quadruped' were -traced, or the stages 

 of man's physical progress in early life reported upon, much the same 

 course of development as that described in the case of the chick would 

 be chronicled. We should see segmentation of the quadruped-germ 

 (Fig. 93, A, B, c, D), as in the lancelet ; we should note the formation 

 of a blastoderm and its three layers, of a primitive groove, of a noto- 

 chord (Fig. 94, ch\ of three brain- vesicles ( Fig. 96, fb, mb, hb}, of visceral 

 or branchial clefts (Fig. 97,,^), and of other structures similar to those 

 of the chick. Only in the latest stages, should we be able to trace 

 the appearance of the higher features of the quadruped or mammal 

 as distinguished' from those of the bird. Human development, so far 

 as has been traced, runs parallel with that of lower forms of life, and 

 exhibits the- " morula" stage (Fig. 93, D) equally with the sponge 

 (Fig. 87, 3), sea-squirt (Fig. 89, 3), or lancelet (Fig. 92, 3). Man's de- 

 velopment is in truth but an epitome condensed and modified, it may 

 be, but still a recapitulation of that of lower forms of life. Thus it 

 is no mere supposition, but a weighty physiological fact, that through 

 flitting and successive stages, which exactly repeat and represent per- 

 manent forms in lower life, man finally attains to be the " paragon of 

 animals." And thus, also, the community of type and general struc- 

 ture which man shares with the lowest fish is demonstrated anew by 

 the marvellous history of the manner in which that type is evolved, 

 alike in its lower and higher phases. 



The marshalling of facts to form generalisations, and the stringing 

 of these facts upon the thread of a connected history, 13 a duty which 



