THE GENEALOGY OF SHIPS. 309 



steam-tug, strong, indeed, like a rhinoceros, but holding 

 a low position in its class, a position little elevated above 

 that of a sailing-craft, and, in fact, incorporating all the 

 fundamental ideas of that craft, except that engine is 

 substituted for sail. The ferry-steamer is an improve- 

 ment, but as the tug responds to a peculiar demand, so 

 does the improved steamer; and each is the product of 

 circumstances. The tug is all muscle; the ferry-boat 

 is broader-shouldered, for bearing its load instead of pull- 

 ing it. The river steamer is the outcome of the fluvia- 

 tile duties of the ferry-boat. It arose in the epoch be- 

 fore the ocean-steamer, and must, therefore, have been 

 its progenitor; and the Great Eastern is the "Kentucky 

 Giant" of the whole class. Only this and nothing more, 

 but there have been divergences from the straight line of 

 descent, as we get aberrant mammals like the ornitho- 

 rhynchus, the armadillo and the sea-lion. The urgency of 

 surrounding conditions has called into existence such col- 

 lateral types as Stevens' battery and the steam dredge, 

 all showing, by their fundamental plan of structure, 

 derivation from the ancestral puffer. 



I think the idea must protrude visibly. It is not that 

 these forms in naval anatomy exhibit an evolution of the 

 idea of a water-vehicle. It is not that they all sustain 

 relations of fundamental plan to each other. It is not 

 that they all show adaptation to special ends, suggesting 

 to the minds of the credulous the notions of design and 

 designer. This is the focus of the logic : They have all 

 descended from an ancestral saw-log, and this appearance 

 of common plan is not a plan, but only a family resem- 

 blance necessitated by the laws of inheritance; this gradual 

 improvement comes from the struggle for existence, where- 



