i8 HISTORY OF THE HUMAN BODY 



tinual divergencies on the part of animals, as they seek new 

 environments in this way, produce the numerous divergent 

 branches, the relative time of the divergence being expressed 

 by the position of the intersection and the amount of the mod- 

 ification by the length of the line. 



II. Although animal forms are not related to one another 

 as members of a single linear series, they yet form a con- 

 tinuum, and any two living forms, however great the struc- 

 tural difference between them, are connected to one another 

 by a continuous chain of animals, a connection which will 

 become apparent by tracing the lines backwards along the 

 ancestral course of each until they meet at their earliest com- 

 mon ancestor. 



Thus, in tracing the relationships of 9 and 14, neither form 

 is ancestral to the other, but both arose from the common 

 ancestor 7, back of which their history is identical. As the 

 ancestral forms are now wholly extinct, they are no longer 

 available for study save when found in the fossil state, but 

 their place may often be supplied by modern forms which are 

 but little modified from the condition of the actual ancestors. 

 Thus the recent forms 8a and 7a are almost as useful in re- 

 producing this part of the phylogenetic history as 8 and 7 

 would be, and through them the inter-relationship of 9 and 

 14 may be readily traced. This may be stated as a third law: 



III. Although the actual ancestral forms lying at the fork- 

 ing of the branches no longer exist and have seldom been 

 found in a fossil state, many clews of their structure may be 

 obtained by the study of those of their descendants which 

 have retained most completely the ancestral environment, and 

 which have, therefore, kept many or most of the ancestral 

 characteristics. 



Thus in studying the relationships and comparing the struc- 

 ture of two such divergent forms as 32 and 34, the living form 

 300 would be of the greatest assistance, as it would enable 

 the investigator to see what was the common structural 

 heritage from which, through two lines of modification, the 

 two forms in question have developed. 



IV. Among the fossil remains of extinct forms which geo- 



