CRUSTACEA AND MOLLUSCS 243 



organs could not have had a selective value from the 

 first. Even such complicated structures as the wings of 

 birds and insects could arise gradually, as we saw in the 

 fourth chapter, and were clearly useful from the start. 

 Further, it has been rightly pointed out that sudden 

 crises may occur in animals after periods of rest. 

 During these the utmost demands are made on the bodily 

 and mental constitution of the animals in the general 

 devastation, as the stronger often fall upon the weaker 

 members of the same species. Large numbers of 

 animals perish in a severe winter, and we may assume 

 that even a slight variation in the thickness of the fur or 

 the feathers may save an animal from destruction. It 

 has also been said with truth that in the migratory birds, 

 for instance, slight advantages in the power of flight 

 may do much, as the effects accumulate in the course of 

 the long journey. Cyclists know that if they are to win 

 a race every small part of their machine must be as light 

 as possible. 



It is often difficult to imagine how an organ can have 

 arisen by natural selection, yet we detect a means on 

 further reflection. The origin of the wings and 

 of the snail's shell seemed for a long time to be a 

 hopeless enigma, but the solution was at length dis- 

 covered. If we can find the answer to these really 

 difficult questions, we may hope to do the same for the 

 remaining problems in connection with the evolution of 

 animals or organs. But above all we must not at once 

 throw the blame on the theory of selection when we 

 cannot find the solution, but must remember the 

 imperfect condition of our knowledge. We cannot 



