ISO THE ORIGIN OF SPECIES 



instrument might thus be formed as superior to one of glass, as 

 the works of the Creator are to those of man? 



MODES OF TRANSITION 



If it could be demonstrated that any complex organ existed, 

 which could not possibly have been formed by numerous, suc- 

 cessive, slight modifications, my theory would absolutely break 

 down. But I can find out no such case. No doubt many organs 

 exist of which we do not know the transitional grades, more espe- 

 cially if we look to much-isolated species, around which, accord- 

 ing to the theory, there has been much extinction. Or again, if we 

 take an organ common to all the members of a class, for in this 

 latter case the organ must have been originally formed at a remote 

 period, since which all the many members of the class have been 

 developed; and in order to discover the early transitional grades 

 through which the organ has passed, we should have to look to 

 very ancient ancestral forms, long since become extinct. 



We should be extremely cautious in concluding that an organ 

 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 func- 

 tions; 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 selec- 

 tion 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, how- 

 ever, 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 indi- 

 vidual the same function, and this is an extremely important 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 



