230 GEOLOGICAL BIOLOGY. 



not precede it. Multiplicity of laborers is a condition neces~ 

 sary to the division of labor, and the organic co-operation of 

 separate parts. 



Comparison between Embryonic Development and Succession of 

 Ancestors. Prenatal or embryonic development of higher ani- 

 mals may pass through stages similar to those expressed in 

 the mature form of lower animals which are supposed to be in 

 the line of descent of the former: as an example, the Mam- 

 malian embryo develops gill-arches, which are characteristic 

 of the mature stage of fishes: but in the embryo of the mam- 

 mal this feature appears in the earlier embryonic life, and is lost 

 as development proceeds. Much has been made by embry- 

 ologists and also by systemists of this embryonic calling back 

 to supposed ancestral characters; but in deriving conclusions 

 from these facts it must be remembered that, since the organs 

 are neither fully co-ordinated nor completed for action in the 

 individual embryo, and that not until the natal stage is past, 

 the likeness of these characters to the mature parts of sup- 

 posed ancestors is rather a likeness in the plan or course of 

 development than in the results of the development. The 

 course of the development may be alike in two organisms ; 

 that is, the steps by which the morphological features may be 

 attained may be according to the same plan, and indicate a 

 fundamental affinity, which is less evident or quite lost in the 

 mature animal. However, it is not a necessary inference that 

 in the embryonic development we will be able to recognize 

 the relationship to an ancestral mature form. Changes, such 

 as abbreviation, or a different course of development, of the 

 embryo, can be assumed to be indicative of phylogeny only 

 in case environment was the determining cause of their origi- 

 nal appearance. If there be an evolution in these modes of 

 differentiation, as there is an evolution in the final product, 

 the resultant differences may be determined by other laws. 



Muscular Motion or Specialized Motion, and Locomotion. 

 The preparation for motion of the organism in definite direc- 

 tion is exhibited in the differentiation of the head as the 

 oral end of a moving organism. Next, it is seen in the differ- 

 entiation of the assimilating cavity into a tube, the enteron, 

 with separate entrance and exit, for the materials of assimi- 



