MODMS OF TRANSITION. 175 



among the_ lower anima*ls of the same organ performing at 

 the same time wholly distinct functions; thus in the larva 

 of the dragon-fly and in the fish Cobites the alimentary 

 canal respires, digests and excretes. 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 advantage were 

 thus gained, the whole or part of an organ, which 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 produce one kind alone, a great change 

 would be effected with comparative suddenness in the 

 character of the species. It is, however, probable that the 

 two sorts of flowers borne by the same plant were originally 

 differentiated by finely graduated steps, which may still 

 be followed in some few cases. 



Again, two distinct organs, or the same organ under two 

 very different forms, may simultaneously perform in the 

 same individual the same function, and this is an extremely 

 important means of transition: to give one instance — there 

 are fish with gills or branohiaB that breathe the air dis- 

 solved 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 in- 

 stance from the vegetable kingdom: plants climb by three 

 distinct means, by spirally twining, by clasping a support 

 with their sensitive tendrils, and by the emission of aerial 

 rootlets; these three means are usually found in distinct 

 groups, but some few species exhibit two of the means, or 

 even all three, combined in the same individual. In all 

 such cases one of the two organs might readily be modified 

 and perfected so as to perform 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-bladder in fishes is a 

 good one, because it shows us clearly the highly 

 important fact that an organ originally constructed for 

 one purpose, namely, flotation, may be converted into 



