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THE AMERICAN NATURALIST [Vol. XLIX 



from Berkeley and La Jolla are assigned by Osgood to 

 the same subspecies " gambeli," although, as I shall point 

 out, there are certain slight differences of type between 

 the two. 



Now, as to characters, I have made 14 measurements of 

 each completely measured mouse. Certain color char- 

 acters, not capable of quantitative expression, have also 

 been taken into consideration. I shall first consider the 

 measurable parts. I must introduce this discussion by 

 stating that my comparisons are entirely between animals 

 of the same body length. When I say that P. m. rubidus 

 has a longer tail than sonoriensis, I mean that this is true 

 for mice of equal size. Owing to the impracticability of 

 giving you a mathematical justification of all the steps 

 which I have taken, I will ask yon to credit me with a 

 knowledge of the more elementary statistical methods. 5 

 I must also explain that I have thus far failed to kill 

 and measure many animals from which I shall before 

 long have full data. At present these are being retained 

 for breeding purposes. Hence my series of measure- 

 ments, in certain cases, is very small. 



To present these subspecific characters briefly, I may 

 say that, in respect to tail and foot length, rubidus stands 

 in a class by itself. It does not require the trained eye of 

 a systematist to detect the fact that this northern race 

 has conspicuously longer tail and feet. In the case of 

 the tail, this difference is due almost wholly to a differ- 

 ence in the length of the individual vertebrae, not to an 

 increase in the number of these. The other three races 

 {sonoriensis and the two lots of gambeli) show no statis- 

 tically certain differences in either of these characters. 

 P. m. rubidus likewise has a significantly greater skull 

 length and probably also a greater cranial capacity. 7 



work. Some of these were pn-.-nre.l to the meeting in the form of graphs. 

 • The same was found to be true of the artificially induced modifications 



-Determined by suitably cleaning and then desiccating the skulls, and 

 weighing the volume of mercury which just filled the cranial cavity. 



