TREE-LIKE GROUriNG. 35 



tree-like grouping of the members of the same family. 

 The reciprocal relations of the various families can- 

 not be represented in a simple line ; though in former 

 days more importance was attributed to the general 

 indications of the relative value of the types. On the 

 other hand, descriptive zoology had long been compelled- 

 to devise tables of affinity for the systematic subdivisions, 

 descending even to species according to the criterion of 

 anatomical perfection ; and these found expression only 

 in diagrams of highly ramified trees. Branches ap- 

 peared which terminated after a brief extension ; others 

 are greatly elongated with numerous side branches ; in 

 every branch characteristic phenomena and series are 

 made manifest. 



Let us attempt it with the Vertebrata, for example. 

 Even with the fishes we fall into great perplexity ; 

 which to place at the end as being the highest. But 

 take which we will, the sharks or our teleostei, the am- 

 phibians cannot be annexed in a direct line, nor does the 

 elongated branch line of the latter merge, as might be 

 imagined, into the rejDtiles. The birds, on their side, 

 offer a sharp contrast to the mammals, and this separa- 

 tion and divergence extend to all the subdivisions. We 

 must figuratively represent family branches, clusters of 

 genera, and tufts of species, which latter ramify into 

 sub-species and varieties. With this representation of 

 the tree-like distribution of the system, we shall gladly 

 revert to the comparison of the members of different 

 types, with reference to their functional value. The bee 

 in itself is manifestly a far more complex organism than 

 the lowest fish-like animal, the lancelet ; and in these two 

 we compare a low form of a high type, and a high form 

 4 



