NATURAL SELECTION 395 



Objections to the Theory. Darwin recognized many objections 

 to his theory, and a few others have been added in later times. 

 The sahent objections may be summed up as follows: 



L If species have descended from other species by fine grada- 

 tions, what has become of the many transitional forms? 



2. Can the production of relatively simple organs and such 

 complex structures as the eye both be explained on this basis? 



3. Can instincts be acquired and modified through natural se- 

 lection? 



4. How can the sterility of hybrids be accounted for? 



5. How can the development of neuter castes be accounted for? 



6. Can natural selection account for the occurrence of vestigial 

 structures and overspecializations when they are of indifferent 

 value to the organism? 



Most of these objections have been adequately met, but a few 

 indicate limitations of the theory. 



Answers to Objections. Transitional Forms. Darwin's an- 

 swer to the first objection was: "As natural selection acts 

 solely by the preservation of profitable modifications, each new 

 form will tend in a fully stocked country to take the place of, and 

 finally to exterminate, its own less improved parent-form and 

 other less favored forms with which it comes into competition. 

 Thus extinction and natural selection go hand in hand. Hence, if 

 we look at each species as descended from some unknown form, 

 both the parent and all the transitional varieties will generally 

 have been exterminated by the very process of the formation and 

 perfection of the new form." The failure of these intermediates 

 to persist in abundance in the geological record is readily explained 

 by the incompleteness of that record; they do exist in small 

 numbers, and transitional individuals occur among some Uving 

 creatures. 



Complex Organs. The production of complex organs was not 

 an insurmountable difficulty to him, although he recognized the 

 impossibility of explaining in detail just how the gradual elabora- 

 tion of such an organ as the eye might have occurred. As a 

 matter of fact, the eyes of modern insects range from a state 

 of development which makes possible merely the reception of light 

 stimuli and the determination of the direction whence they come, 

 to the elaborate compound eyes of higher orders (Fig. 206, compare 

 with Fig. 62 and 63). The very existence of stages of development 



