Blood Affinities 115 



of rabbit sensitized to dog protein, we shall find this serum 

 forming precipitations in varying degree with each sample 

 — but more in proportion as the animal being tested is more 

 closely related to the dog. 



Blood Affinities 



By extending and refining this method, special students 

 have been able to tell us with assurance that this principle 

 of chemical affinity as shown by the response of serum from 

 sensitized animals runs parallel to genetic affinity as judged 

 by structure. That is to say, the more alike animals are in 

 their structure, the more alike are they also in their chemical 

 composition. The more alike plants are in their form and 

 plan of structure, the more alike are they also in the fine 

 gradations of chemical constitution. A very extensive study 

 of ferns, quillworts and club mosses, and other types of plants 

 whose classification has involved many difficulties, reveals a 

 striking parallelism between the structural resemblances, the 

 developmental histories and the fossil records, on the one 

 hand, and the serum tests on the other. The closeness of 

 genetic relationships was judged by the intensity of the re- 

 actions obtained between the serums of sensitized animals 

 and the protein extracts of a dozen representative species. 

 The more recent work is at all points consistent with the 

 work of different investigators who have used the serum tests 

 in the study of plant relationship. 



In comparing other groups, the student of comparative 

 anatomy is frequently puzzled^ by finding a form whose 

 relationships are obscure, or rather whose relationships branch 

 in two or more directions. An example of this is seen in the 

 so-called horseshoe crab, which is sometimes considered to 

 be a kind of crab for good enough reasons, although it is 

 fundamentally different from the crabs and lobsters in many 

 important respects. It has no close living relatives, but in 

 former times the seas were full of many species of trilobites 

 (see page 30, Fig. 3). On structural grounds the horseshoe 



