observed is the resultant of the combined action of many causes. As the complex phenomena are analyzed into their simple (or, perhaps better, simpler) components correlation passes over into causation. It is, however, a great aid in making such analyses to have a method of measuring exactly the degree of correlation which exists between observed phenomena or characters. Of such methods current biometrical technique furnishes a great wealth. It is possible now to determine the degree of correlation or association existing between not only physically measurable characters, but also between qualitative characters not capable of precise measurement.
IV.
Let us now turn our attention to the consideration of the limitations of biometry as a branch of biological science. There is, I think, fundamentally but one such limitation. This arises out of the fact, already mentioned, that biometrical methods of research are, in last analysis, strictly and purely « descriptives » in character. There are but two general ways of acquiring and formulating a knowledge of natural phenomena. These are the descriptive method on the one hand, and the experimental method on the other hand. Biometrical methods belong in the first of these categories. The only thing which they are able to do is to furnish a description, in quantitative terms, of existing phenomena. This does not, of course, imply that they are not useful aids in experimental investigations. Indeed it is just here that, in the writer's opinion, biometry finds in general its highest usefulness in biology. It is only desired to emphasize the fact that biometric methods are per se purely descriptive, and have the limitations implied thereby.
In actual biometric work the importance of keeping clearly in mind the limitations and precise significance of the methods used is great. To reach biologically significant results one must understand clearly just what is being measured and what the utility of that measurement really is. Failure to do this is bound to result in confusion of thought. No description has any significance unless the thing described
