Divergence in Human Development 141 



These parallels in divergence during development call 

 for an explanation in terms of a common set of causal fac- 

 tors. We may suppose that living matter, present in the liv- 

 ing c^g of any animal, must pass through certain stages in 

 order to become an adult, regardless of how any species 

 originated. Mammals, for example, must have their embryos 

 behave during development just as they do because they 

 must at each successive stage be adapted to living within the 

 womb of the mother, to getting nourishment from the 

 mother's blood, to breathing and elimating wastes by means 

 of the mother's blood. Many of the structures and proc- 

 esses observed in the developing fetus are indeed remarkably 

 adapted to this specialized mode of life. 



It may satisfy many to say that the embryos develop as 

 they do because that is the most suitable way of passing from 

 the one-celled c^^ to the billion-celled adtilt. Such an ex- 

 planation, however, leaves open the major problem. Why 

 does the embryo start to produce structures that serve no 

 function whatever, either during development or later, many 

 such structures that are in fact later lost? Why must the 

 very young person, for example, grow a tail only to lose it 

 again before meeting his mother face to face? Why must 

 the embryos of air breathers start out to elaborate water- 

 breathing organs and then change their course and lose most 

 of the evidence of the abortive attempt before they begin to 

 breathe on their own? 



The evolutionist's explanation for these facts is found 

 in the supposition that species arose by progressive divergence 

 from ancestral types. The development is a manifestation 

 of heredity, not in the sense of Ernst Haeckel's extreme doc- 

 trine of recapitulation, but in the sense that in so far as two or 

 more species are actually related through common descent, 

 they will show a similarity in the stages of development as 

 well as in the adult form. The more closely related two in- 

 dividuals are the longer will they remain alike in progressing 

 from the earliest indistinguishable stages. The less closely 

 related two individuals are, the earlier in their development 

 will differences between them appear. 



