Chemical Differences iiy 



and of antitoxins, and it is true of numberless substances 

 for which as yet we have not even any names. For example, 

 bacteria of a given kind will cause disease in some animal 

 but not in other animals. We say that a given species is 

 susceptible to a given disease, whereas other species are im- 

 mune. But we find also that this susceptibility or immunity 

 varies in degree, and that in a given series of animals the 

 susceptibility to disease (or immunity) will vary in almost 

 exact proportion to the degree of relationship as judged by 

 structural characters. We share our diseases with our near- 

 est relatives. We seldom succumb to the disease of remote 

 types of animals. These differences are not absolute but 

 they are significant only because they parallel the distribution 

 of other differences. 



We see a similar parallelism in the way insect pests or 

 parasitic fungi attack plants. The potato bug (or rather 

 beetle) has caused havoc to cultivated potato plants, leaving 

 the rest of the farmer's crop undisturbed. You cannot tempt 

 these animals with cabbages or rose leaves. They will, 

 however, eat the leaves of wild plants related to the 

 potato. There are hundreds of insects that confine them- 

 selves to certain species of plants for food. They can be 

 diverted from these plants in most cases only by closely 

 related plants. 



In more recent times the study of blood chemistry has 

 revealed the presence of blood " types " among human beings, 

 and these variations are shown to be inherited. We can ap- 

 ply the principles of classification not only to the structure 

 of living things, but also to th^ir chemical qualities, and we 

 infer relationships from the latter on the basis of degrees 

 of similarity. 



A further line of study in which chemical constitution 

 is found to be significant is in the differentiation of types 

 of bacteria. These organisms are so similar and so simple 

 in structure that very few characteristics can be recorded 

 about their form, size and appearance. Bacteria from many 

 different sources are quite indistinguishable under the micro- 



