270 SCIENCE PROGRESS 



we see all degrees of blending, and there seems no possibility of 

 expressing it as a sorting out of minor characters. 



Let us, therefore, look at the matter from another standpoint. 

 Let us look at Mendelian inheritance, not as the normal form of 

 inheritance, but as a modified form of mutual infertility. Men- 

 delian inheritance is the characteristic of stocks that do not truly 

 blend. The various varieties emerge from the process of inter- 

 crossing practically unchanged. This clearly tends to fix the 

 types of the crossing varieties. It accomplishes, in a different 

 way, the same purpose as the mutual infertility of allied species. 

 Does it not, therefore, seem a plausible suggestion that this is 

 merely a step on the road towards species formation, that the 

 practically complete blending of ordinary inheritance, the 

 emergence of unaltered types from the process of Mendelian 

 crossing, the partial infertility of the equidse, the entire mutual 

 infertility of other allied species, may be but parts of a continuous 

 process, the formation of distinct physiological species? 



This is, of course, merely a speculation, and will require 

 considerable confirmation before it is possible to make use of it, 

 but I put it forward as an illustration of the necessity of avoiding 

 undue dogmatism concerning the possible methods of deter- 

 mining geologic time. Because biological data have, as yet, 

 thrown no light on this subject, we must not be too ready to 

 assume that such may not be available in the future. It is 

 therefore, of interest once more to raise the question : has a 

 truly infertile physiological species ever been formed within the 

 time of human observation, or has, indeed, any series of varieties 

 been formed which will intercross in a definitely determinable 

 Mendelian manner? Changes of form are produced quickly, 

 whether by selective breeding or by* change of conditions. But 

 the problem of physiological species is still unsolved and it may 

 be that a great lapse of time is required to form them. 



D. Fossils as an Index of Geologic Time 



Suggestions such as those referred to in the last section are 

 problems for future research. For the present, pure biological 

 methods, particular as well as general, have yet to be found. 

 We shall, therefore, now glance at the more obvious line of 

 advance found in the co-ordination and correlation of biologic 

 and geologic data. In its broad outlines, the method has been 



