Evolution and Taxonomy 39 



forms have survived whose parts were best fitted to perform 

 their functions. 



Obviously there are certain difficulties in the carrying out 

 of this plan. But the measure of our success in determining 

 the affinities of the organisms studied, will depend largely on 

 our ability to overcome these difficulties. 



Among the difficulties encountered is the fact that usually 

 our classification must be based largely on a study of living 

 forms ; for in most cases the aid to be derived from Palaeon- 

 tology is comparatively slight. But although the record pre- 

 sented by fossils is very fragmentary, fortunately there are 

 many living forms which are comparatively slightly special- 

 ized. And these will serve to give an idea of the stem form 

 of the group. 



Thus to carry out the plan suggested, the zoologist or 

 botanist, if he is forced to work only with living animals or 

 plants, will select from the group to be studied the most gen- 

 eralized type before him, and then trace out the different ways 

 in which this type has been modified in the more specialized 

 forms. 



If the group studied be a large one, the probabilities are 

 that instead of a single primitive type, several generalized 

 forms will be found, each representing more or less approxi- 

 mately the stem form of a distinct line of development ; and a 

 comparative study of these different forms will be necessary 

 in order to obtain an idea of the structure of their common 

 ancestor. 



But how shall one go to work to select from a large number 

 of forms those that are to be considered the more generalized ? 

 The higher animals and plants are such complex organisms 

 that it is not an easy matter to determine the relative degree 

 of specialization of two distinct forms. The problem is also 

 complicated by the fact that even the more generalized forms 

 may present specializations peculiar to themselves. 



Numerous examples will occur to any systematist of forms 

 which as a whole are comparatively generalized, but which in 

 some respects are highly specialized, being, as has been ex- 

 pressed by some writers, " sidewise developed." It is es.sen- 



