174 MODES OF TRANSITION, 



every part of this layer to be continually changing slowly 

 in density, so as to separate into layers of different densi- 

 ties and thicknesses, placed at different distances from each 

 other, and with the surfaces of each 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 degree, 

 tends to produce a distincter image. We must suppose 

 each new state of the instrument to be multiplied by the 

 million; each to be preserved until a better one is pro- 

 duced, and then the old ones to be all destroyed. In 

 living bodies, variation will cause the slight alteration, 

 generation will multiply them almost infinitely, and natural 

 selection will pick out with unerring skill each improve- 

 ment. Let this process go on for millions of years; and 

 during each 5^ear 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 TKAKSITI03S". 



If it could be demonstrated that any complex organ ex- 

 isted, which could not possibly have been formed by nu- 

 merous, successive, 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 especially if we look to much- 

 isolated species, around 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 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 grada- 

 tions of some kind. Numerous cases could be given 



