0. — GEOLOGY. 63 



a horizontal section across the tree of life — that is to say, with an assem- 

 blage of approximately contemporaneous forms — or even with a number 

 of such horizontal sections, so long were we confined to simple descrip- 

 tion. Any attempt to frame a causal connection was bound to be 

 speculative. Certain relations of structure, as of cloven hooves with 

 horns and with a ruminant stomach, were observed, but, as Cuvier him- 

 self insisted, the laws based on such facts were purely empirical. 

 Huxley, then, was justified in maintaining, as he did in 1863 and for 

 long after, that a zoological classification could be based with profit on 

 * purely structural considerations ' alone. ' Every group in that [kind 

 of] classification is such in virtue of certain structural characters, which 

 are not only common to the members of that group, but distinguish it 

 from all others ; and the statement of these constitutes the definition of 

 the group.* In such a classification the groups or categories — from 

 species and genera up to phyla — are the expressions of an arbitrary in- 

 tellectual decision. From Linnaeus downwards botanists and zoologists 

 have sought for a classification that should be not arbitrary but natural, 

 though what they meant by ' natural ' neither Linnaeus nor his succes- 

 sors either could or would say. Not, that is, until the doctrine of 

 descent was firmly established, and even now its application remains 

 impracticable, except in those cases where sufficient proof of genetic 

 connection has been furnished — as it has been mainly by palaeontology. 

 In many cases we now perceive the causal connection ; and we recognise 

 that our groupings, so far as they follow the blood-red clue, are not 

 arbitrary but tables of natural affinity. 



Fresh difficulties, however, arise. Consider the branching of a tree. 

 It is easy to distinguish the twigs and the branches each from each, 

 but where are we to draw the line along each ascending stem ? To con- 

 vey the new conception of change in time we must introduce a new set 

 of systematic categories, called grades or series, keeping our old cate- 

 gories of families, orders, and the like for the vertical divisions between 

 the branches. Thus, many crinoids with pinnulate arms arose from 

 others in which the arms were non-pinnulate. "We cannot place them 

 in an Order by themselves, because the ancestors belonged to two or 

 three Orders. We must keep them in the same Orders as their respec- 

 tive ancestors, but distinguish a Grade Pinnata from a Grade Impin- 

 nata. 



This sounds fairly simple, and for the larger groups so it is. But 

 when we consider the genus, we are met with the difficulty that many 

 of our existing genera represent grades of structure affecting a number 

 of species, and several of those species can be traced back through 

 previous grades. This has long been recognised, but I take a modern 

 instance from H. F. Osborn's ' Equidae ' (1918, Mem. Amer. Mus. 

 N.H., n.s. n. 51): 'The line between such species as Miohipptt-s 

 (Mesohippus) meterilophus and M. hroxhyatylus of the Leptauchenia 

 zone and M. (Mesohipptis) intermedius of the Protoceras zone is purely 

 arbitrary. It is obvious that members of more than one phylum [i.e.. 

 lineage] are passing from one genus into the next, and MesMppus 

 metetdophus and M. hrachystylus may with equal consistency be 

 referred to Miohippti,s.' 



