70 INTRODUCTION TO EVOLUTION 



that this conclusion concerning the ancestry of mammals would have been 

 reached even though mammalian embryos had not resembled fish embryos 

 and reptilian embryos. Studies of anatomy and of the fossil record would 

 by themselves have led to the conclusion; embryonic recapitulation adds 

 welcome confirmatory evidence. In this and in many other instances 

 embryology confirms conclusions based on other lines of evidence. When 

 other lines of evidence are lacking, or contradictory, to what extent is it 

 safe to draw conclusions as to ancestry from embryology alone? With due 

 recognition of factors noted above that modify recapitulation, can we 

 nevertheless use the embryonic record as a means of reaching conclusions 

 concerning the nature of ancestors that are otherwise unknown? 



One case in which such conclusions have been generally drawn, al- 

 though not without dissent, is that of the origin of many-celled animals, 

 the metazoa. Most biologists are agreed that the first animals were single- 

 celled organisms, protozoa, and that many-celled animals evolved from 

 these single-celled ones (see Boy den, 1953, for a dissenting view, how- 

 ever). But if so, what were the transitional stages in this evolution? Many, 

 probably most, biologists conclude that embryonic development gives us 

 clues to the answer. 



Earlier in the chapter we noted that early embryonic developments of 

 widely diverse organisms are homologous, that is, are so similar that a 

 typical sequence of stages can be drawn (Fig. 4.2). Do these stages repre- 

 sent a case of recapitulation; do they give us clues as to the stages the 

 metazoa passed through in their evolution from the protozoa? Striking 

 parallels can be drawn between these stages and simple organisms living 

 at the present time. In Fig. 4.15 the first vertical column represents the 

 stages in typical embryonic development; the second column contains rep- 

 resentative modern organisms that show comparable structures. In the top 

 squares of each column are single cells, the ovum on the one hand, typi- 

 cal protozoa on the other. If this is a case of recapitulation, we have in the 

 fact that all organisms begin life as a single cell (the fertilized ovum) con- 

 firmatory evidence that metazoa evolved from protozoa. 



In the second squares of both columns of Fig. 4.15 we have small ag- 

 gregates of cells: cleavage stages of embryonic development on the left, 

 simple colonial organisms on the right. Gonium and Pandorina are rep- 

 resentative organisms that consist of colonies of cells. They are frequently 

 called colonial protozoa but since they possess chlorophyll they may be 

 classed as plants. Perhaps they are best classified as Protista without as- 

 signing them to either the plant or the animal kingdom. At any rate it is 

 the general level of organization rather than exact relationships that con- 



