1S2 MODES OF TRANSITION. Chap. VI. 



all the mail)'- members of the class have been developed ; and, 

 iu order to discover the early transitional grades through 

 which the organ has passed, Ave should have to look to very 

 ancient ancestral forms, long since become extinct. 



We should be extremely cautious in concluding that an or- 

 gan could not have been formed by transitional gradations of 

 some kind. Numerous cases could be given among the lower 

 animals of the same organ performing at the same time wholly 

 distinct functions ; thus the alimentary canal respires, digests, 

 and excretes, in the larva of the dragon-fly and in the fish Co- 

 bites. In the Hydra, the animal may be turned inside out, and 

 the exterior surface will then digest and the stomach respire- 

 In such cases natural selection might specialize, if any advan- 

 tage were thus gained, the whole or part of an organ, Avhich 

 had previously performed two functions, for one function alone, 

 and thus by insensible steps greatly change its nature. Many 

 plants are known which regularly produce at the same time 

 differently-constructed flowers ; and if such plants were to pro- 

 duce one kind alone, a great change would in some cases be 

 effected in the character of the species. It can also be shown 

 that the pi'oduction of the two sorts of flowers by the same 

 plant has been effected by finely-graduated steps. Again, two 

 distinct organs in the same individual may simultaneously per- 

 form the same function, and this is a highly-imj^ortant means 

 of transition : to give one instance — there are fish with gills 

 or branchiae that breathe the air dissolved in the water, at the 

 same time that they breathe free air in their swim-bladders, 

 this latter organ being divided by highly-vascular partitions, 

 and having a ductus pneumaticus for the supply of air. To 

 give another instance from the vegetable kingdom: plants 

 climb l^y three distinct means, by spirally twining, by clasping 

 a support with their sensitive tendrils, and by the emission of 

 aerial rootlets ; tliese three means are usually found in distinct 

 groups, but some few plants exhibit two of the means, or even 

 all three, combined in ttie same individual. In all such cases 

 one of the two organs for performing the same function might 

 be modified and perfected so as to ])erf()rm all the work, being 

 aided during the progress of modification by the other organ ; 

 and then this other organ might be modified for some other 

 and quite distinct purpose, or be wholly obliterated. 



The illustration of the swim-l)ladder in fishes is a good one, 

 becausi? it shows us clearly the higher important fact that an 

 organ originally constructed for one purpose, namely flotation, 



