GENERAL LAWS OF ANIMAL STRUCTURE. 247 



teristic of all living things. We have already given this 

 illustration on page 22. All tissues have a common 

 origin in unmodified cell structure. But in the differ- 

 entiated tissues of the higher animals this common 

 origin is so obscured by adaptive modification for func- 

 tion that it can not be made out except by extensive 

 comparison in the animal scale and in the embryonic 

 scale. Now the object of this chapter is to trace the 

 homologies or the common origin in animal structure in 

 spite of the obscurations produced by modification of 

 parts adapting it to various functions, and of the whole 

 organism adapting it to various places in the economy of 

 Nature. 



But the question occurs : How far can we trace 

 homology ? Analogy can be traced throughout the ani- 

 mal kingdom, because it is based on function. Doubt- 

 less, also, homology or common descent must exist 

 throughout, but it is not traceable with certainty. The 

 obscurations of adaptive modification completely oblit- 

 erate the evidences of common origin except in animals 

 not too far separated in character. How far, then, can 

 we satisfactorily trace it ? We answer : Only through 

 the primary divisions or departments. And, conversely 

 from the morphological or evolutional point of view, this 

 ability to trace common origin is the true ground of pri- 

 mary divisions. From this point of view Agassiz's four 

 primary branches from the protozoan trunk finds much 

 justification. 



Thus, then, we would say, the structure of all verte- 

 brates is exactly what it would be if they all came by 

 descent with modification from one primal vertebrate; 

 and therefore we must believe they did so come. All 

 articulates — i. e., arthropods and annulates — for like rea- 

 son, came from a primal form of articulate by adaptive 

 modification. Similarly all mollusca came by descent 



