74 PRINCIPLES OF ANIMAL BIOLOGY 



cells, when they can be distinguished at all, and sometimes contain granules 

 of a peculiar sort. Finally, to complete the comparison, development of 

 the embryo need be followed only a short way to ol^serve differentiation 

 of the somatic cells into at least as many kinds as Hydra possesses. 



A Conclusion, and Caution in Adopting It. — The principle of using 

 embryonic development to discover the course of evolution is known as 

 the biogenetic law. According to this generalization, the development of 

 an individual repeats the history of its race. This law is seriouly ques- 

 tioned by many biologists and vigorously opposed by some. Also, the 

 ^■r^f^;:y,-;^!-s'yf"'£r-<:S:^\:-f/:m:::M use of scrics of modern organisms to illustrate 



what may have taken place in evolution must 

 ])e made, if at all, with great care. Both of 

 these comparisons have been made, however, 

 by l)iologists in the past, using the organisms 

 referred to in the preceding pages. The con- 

 clusion to which they lead is that metazoa 

 have arisen through (1) the adherence of pro- 

 FiG. 63. — Posterior end tozoan cclls to fomi a colony, (2) the loss of 

 of developing insect egg. reproductive powers by some of the cells of 

 ductive cells, all others this colony, and (3) the differentiation of these 

 somatic. sterile cells into a number of kinds. These are 



the fundamental steps; the details of cell structure and the general form 

 of the colony are immaterial. 



This conclusion, it will be observed, accepts the colonial and rejects 

 the organismal theory. It rather favors spheroid colonies over the 

 gregaloid type because the modern organisms availal)le for a series of 

 representative types are spherical, and because the blastula of embryonic 

 development is a hollow ball. Many biologists hesitate to recognize 

 these reasons, and reference to them here is in no sense a pronouncement 

 in favor of the mode of origin of the metazoa which they appear to 

 indicate. Nevertheless, that origin is not improbable. And even if the 

 scheme of evolution described should be far from correct, a consideration 

 of it has led to an understanding of the relation of parts to wholes and a 

 glimpse of some of the situations which many-celled organisms have to 

 meet. 



What Is a Colony, What an Individual? — When any change is effected 

 by a number of graduated steps, as the origin of metazoa from simpler 

 organisms must have been, it is difficult to say just wluui any stage that 

 may be named is reached. When, for example, has a metazoon been 

 evolved out of a protozoon? How far must the change go to be recog- 

 nized as having reached that goal? No matter what process led to 

 the metazoa, the answer to this question must be a matter of definitions. 

 If the organismal theory is correct, was the animal with numerous 



