1S96 .] The Formulation of the Natural Sciences. 107 



the family Equidse. When one is tracing the phytogeny 

 of species, he is tracing the descent of the numerous char- 

 acters which define a species. This is a complex prob- 

 lem, and but little progress has been made in it from the 

 paleontologic point of view. Something has been done with 

 regard to the descent of some living species from each other. 

 But when we are considering the descent of a genus, we re- 

 strict ourselves to a much more simple problem, i. e., the de- 

 scent of the few simple characters that distinguish the genus 

 from other genera. Hence, we have made much more pro- 

 gress in this kind of phylogeny than with that of species, espe- 

 cially from the paleontologic point of view. The problem is 

 simplified as we rise to still higher divisions, i. e., to the in- 

 vestigation of the origin of the characters which define them. 

 We can positively affirm many things now as to the origin of 

 particular families and orders, especially among the Mam- 

 malia, where the field has been better explored than else- 

 It is in this field that the unaccustomed hand is often seen. 

 Supposing some phyletic tree alleges that such and such has 

 been the line of descent of such and such orders or families, as 

 the case may be ; soon a critic appears who says that this or 

 that point is clearly incorrect, and gives his reasons. These 

 reasons are that there is some want of correspondence of gen- 

 eric characters between the genera of the say two families al- 

 leged to be phyletically related. And this want of corre- 

 spondence is supposed to invalidate the allegation of phyletic 

 relation between the families. But here is a case of irrele- 

 vancy ; a generic character cannot be introduced in a compar- 

 ison of family characters. In the case selected, the condition 

 is to be explained by the fact that although the families are 

 phyletically related, one or both of the two juxtaposed genera 

 through which the transition was accomplished has or have 

 not been discovered. The same objection may be made 

 against an allegation of descent of some genus from another, 

 because the phyletic relation between the known species of the 

 two genera cannot be demonstrated. I cite as an example the 

 two genera, Hippotherium and Equus, of which the latter has 



