Page - 194 - in The Origin of Species

184 ORIGIN OF SPECIES layer slowly changing in form. Further we must suppose that there is a power, represented by natural selection or the survival of the fittest, always intently watching each slight alteration in the transparent layers ; and carefully preserving each which, under varied circumstances, in any way or in any degree, tends to produce a distincter image. We must sup- pose each new state of the instrument to be multiplied by the million; each to be preserved until a better one is produced, and then the old ones to be all destroyed. In living bodies, variation will cause the slight alterations, generation will mul- tiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions of years ; and during each year on millions of individuals of many kinds; and may we not believe that a living optical 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 ex- isted, which could not possibly have been formed by numer- ous, successive, slight modifications, my theory would abso- lutely break down. But I can find out no such case. No doubt many organs exist of which we do not know the tran- sitional grades, more especially if we look to much-isolated species, round which, according 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 orean 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 amongst the lower animals 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 in-