216 PROCEEDINGS OF THE ACADEMY OF [April, 



than those on which they were determined ; they niay be interpreted 

 to show the mathematically probable further course of development 

 of this species, but they do not show anything with regard to other 

 species. 



In one way this statistical study of variation may be of great 

 service, namely, in regard to the distinguishing of " varieties" or 

 * ' races " of a species. If the degree of variation between the 

 individuals in one and the same portion of the area of distribution 

 of a species be fully as great as the degree of variation between 

 the individuals at the most separated points of this area of distribu- 

 tion, then it is not permissible to speak of ' ' varieties ' ' within such 

 a species. Or, conversely, it is only allov,able to distinguish 

 " varieties," when in a portion A of the geographical area of a 

 species the individuals show a different average of variation than 

 in a portion B. 



Davenport^^ has pointed out that the plotting of variation curves 

 affords a criterion by which varieties may be distinguished from 

 species, thus determining the indices of divergence — " if either of 

 these indices is less than a certain number we have varieties; if 

 above that number, species." But even with this method it is 

 necessary to select a particular character as the " chief differ- 

 ential," and it is just with regard to this point that there is as yet 

 no mathematical precision, and hence this method does not furnish 

 a " precise criterion," though it is certainly a step in the right 

 direction. 



(c) The Components of the Organization, that are of Most 

 Importance in Phylogeny. 



We have seen that the organization as a whole must be considered. 

 The whole, however, can be understood only by synthetically con- 

 sidering the parts of which it is composed. These parts are of very 

 different relative values, from the protoplasmic alveoli or microsome 

 on the one hand, to the organ, organ system and general body 

 plan, on the other. This leads us to the question, which category of 

 parts should be considered in determining phyletic relationship ? 

 Is it immaterial whether we base the classiiicatiou upon cell structure, 

 upon tissue structure, upon organ structure, or upon arrangement of 

 the organs ? Or are there good reasons for using as the basis of 

 examination a particular category of parts ? 



]5 <tjj^ Precise Criterion of Species,'' Science, N. S. VII, No. 177, 1898. 



