164 The Scottish Naturalist. 



the more it generally differs from those now living ; why ancient 

 and extinct forms often tend to fill up gaps between existing 

 forms ; sometimes blending two groups previously classed as dis- 

 tinct into one, but more commonly bringing them a little closer 

 together. The more ancient a form is, the more nearly will it be 

 related to, and consequently resemble, the common progenitor of 

 groups, since become widely divergent. Extinct forms are seldom 

 directly intermediate between existing forms ; but are intermediate 

 only by a long and circuitous course through other extinct and 

 different forms. We can clearly see why the organic remains of 

 closely consecutive formations are closely allied ; for they are 

 closely linked together by generation. We can clearly see why 

 the remains of an intermediate formation are intermediate in 

 character." 



No greater step toward the embodiment of such teaching as 

 this in actual result, can I think be made, than by the amplifica- 

 tion of our zoological and botanical tables of classification by the 

 addition of the animals and plants that lived in past ages. In- 

 numerable problems will no doubt at once suggest themselves ; 

 and there will be very many groups of extinct forms whose precise 

 position on such a classification will be most difficult to determine. 

 These difficulties may however, it seems to me, be in great 

 measure got over by, in the first place, always adopting an 

 arboreal or genealogical form of classification, and by absolutely 

 discarding the unmeaning, if not actually pernicious, tabular 

 arrangement ; and secondly, by the judicious use of such expressed 

 or unexpressed hypothetical links as are made use of, for example, 

 by Balfour in his explanation of the phyllogeny of the Chordata. 

 Probably also were biologists ready to tackle the problem of the 

 structure and affinities of fossil organisms, their knowledge of 

 recent forms might enable them to suggest solutions of difficulties 

 which the experience of geologists might never have led them to 

 guess at. Witness the recent demonstration of the Arachnoid 

 affinities of Limulus and of the extinct Trilobites and Eurypterida. 



