RELATIONSHIP AMONG ORGANISMS. Ill 



a bird and that of a butterfly are likewise analogous in function, 

 but they are not homologous in structure because not derived 

 from a common ancestor. 



Limits of Groups in the Natural System. Although living 

 things can thus be arranged in natural groups which represent 

 deep-seated similarities or relationships due to community of 

 descent, it must not be supposed that these groups are absolutely 

 distinct. On the contrary, the progress of biological knowledge 

 has shown that the various groups of plants and animals merge 

 into one another more or less completely ; that there are con- 

 necting links between them, sometimes so numerous and so nicely 

 graduated that it is impossible to draw any hard and fast line 

 between groups which, broadly considered, appear very distinct. 



Many of the earlier biologists maintained the various groups 

 of plants and animals to possess well-defined boundaries, and de- 

 nied the existence of real connecting links. Cuvier held that all 

 animals without exception might be arranged in four absolutely 

 separate great natural groups the Radiata, Mollusca, Articu- 

 latcij and Vertebrata. By some of his pupils this conception was 

 extended from the four primary groups to their smallest sub- 

 divisions, so that even genera and species were considered immu- 

 table and distinct, in accordance with the view of Linnaeus. At 

 present, however, few if any biologists maintain that the groups, 

 whether smaller or larger, are in reality separated from one 

 another by any well-marked boundaries. Some of them appear 

 to be so at first sight, but in many cases a careful examination of 

 these groups shows them to be connected by intermediate forms, 

 either living or extinct, and we may well hesitate to assume that 

 this is not true of all groups. For example, birds and reptiles 

 form two very distinct natural groups with no living connecting 

 links between them. But the mesozoic rocks contain the fossil 

 remains of animals which must be regarded as intermediate 

 between birds and reptiles; and therefore the boundary -line 

 between them cannot be regarded as impassable. There are 

 strong reasons for believing that species, like the larger groups, 

 have no absolutely fixed limits, but pass into related species by 

 insensible gradations. These gradations are, in some cases, 

 found among living forms, but in others the connection can only 

 be demonstrated by the study of those which have become 

 extinct. The extinction of the connecting forms, in many cases, 



