TREE-LIKE GROUPING. 



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 reptiles. The birds, on their side, 

 oflfer 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 beo 

 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 



