636 



ORGANIC EVOLUTION 



fish to man follow similar pathways, each 

 dropping out in its appropriate place along 

 the line — the fish first, man last. 



Obviously, from what we have already 

 said, the higher embryos will possess fea- 

 tures that are functional in the adults of 

 the lower forms. For example, the human 

 embryo has a set of folds in the neck region 

 in the fourth and fifth week of life. The fish 

 embryo in a comparable stage of develop- 

 ment possesses a similar set of folds; and, in 

 fact, the embryos resemble one another so 

 closely at this early stage that it is difficult 

 to distinguish one from the other (Fig. 

 25-9). In the fish the folds become func- 

 tional gills in the adult, whereas in the 

 human they undergo a whole series of mod- 

 ifications, ultimately becoming a part of the 

 hearing and sovmd-making apparatus as 

 well as a part of the upper respiratory tract 

 (Fig. 25-11). We are reminded once again 

 of tlie fact that, in the long evolutionary his- 

 tory of animals, structures which no longer 

 find a use in the body often become as- 

 signed new tasks and by slight or great 

 modifications become beautifully adapted 

 to the new function. 



There are many other features that ap- 

 pear in the developing human embryo that 

 are tell-tale evidence of our lowly origin. 

 For instance, a well-developed tail (Fig. 

 25-16) makes its appearance in the first few 

 weeks of life but soon vanishes without per- 

 forming any function other than to indicate 

 to us tliat man came from stock which once 

 possessed a tail. We have already noted the 

 presence of a yolk-sac which could hardly 

 have a function since it is devoid of yolk 

 (food) and is not needed because nourish- 

 ment is provided by the placenta. Here 

 again it tells us that some time far back in 

 evolutionary history our ancestors were 

 egg-laying animals in which the embryos 

 did receive their nourishment from a yolk- 

 sac. The later human embryo, along in the 

 fifth month, develops a coat of hair called 

 the lanugo, which is shed before birth, 

 usually in the eighth month of gestation. 



Such hair could not possibly perform any 

 conceivable function but does lead us to 

 believe that at one point in our evolution 

 we possessed a coat of hair as adults, ex- 

 tending back, probably, to some common 

 ancestor of both man and apes. The entire 

 study of both invertebrate and vertebrate 

 embryos supports the theory of evolution; 

 no other explanation serves as well. 



From comparative anatomy 



One of the most striking observations 

 that impresses anyone who studies a large 

 variety of vertebrates is the fundamental 

 likeness of the body architecture. The simi- 

 larities are evident no matter what system 

 is examined — appendages, muscles, diges- 

 tive systems, circulatory systems, or any 

 other. By comparing the appendicular 

 skeletons of the horse, man and the frog 

 (Fig. 25-12) it is obvious that the bones are 

 similar, varying only in emphasis of spe- 

 cific parts of certain bones. In the horse, the 

 forelegs are extended by elongating the 

 equivalents of the wrist and hand bones of 

 the frog and man. This provides the horse 

 with an appendage well adapted for travel- 

 ing at high speeds over soft turf. The same 

 bones modified in different ways have given 

 man a maneuverable appendage which is 

 handsomely adapted to the kind of life he 

 leads. Consider how much of our surround- 

 ings are designed for manipulation with 

 the hand! An appendage not greatly differ- 

 ent from that of man serves the frog satis- 

 factorily in its way of life. Similarly, the 

 hind appendages have been modified to 

 perform specific jobs. The important point 

 to note here is that the fundamental plan is 

 the same; that is, the appendages are ho- 

 mologous, which means that they must 

 have had a common origin. It would be 

 difficult to explain their likenesses on any 

 other basis. 



A study of the muscles in any two verte- 

 brates, man and cat for example (Fig. 25- 

 13 ) , will reveal the same sort of homologies 

 noted in the skeletons. The large, major 



