HIGHWAY OF ANIMAL EVOLUTION 235 



In fact, in any forward step in evolution, it is 

 hardly possible for the biologist to determine which is 

 cause and which is effect. We are rarely, if ever, able 

 to decide whether improvements in any one system of 

 functions precede or follow those in the others. The 



FIGURE 9. FACE OF AN EMBRYO FROG, showing the three pairs of primitive 

 jaws derived from their invertebrate ancestors, and their union to form 

 the jaws typical of the vertebrates. (From Patten, Evolution of the Verte- 

 brates and Their Kin.) 



FIGURE 10. FIGURES ILLUSTRATING THE MODE OF GROWTH OF THE HUMAN 

 FACE. A, human embryo, a little more than a month old, showing the traces 

 of invertebrate jaws in the mandibular, m.d., maxillary, m.x., and pre- 

 maxillar)-, p.mx., arches. B, embryo a little less than two months old, with 

 the preceding parts nearly united. C, mouth of adult, the cupid bow contours 

 showing its elemental structure. From Patten, A Problem in Evolution, 

 Pop. Science, 1913. 



very dependence of one on the other makes their re- 

 spective changes appear to be coincident. But we can- 

 not doubt that whenever any improvement in the serv- 

 ice of one part takes place it demands and receives 

 some compensating service elsewhere. 



