WEALTH OF INSECT LIFE 163 



times and under all circumstances to recognize forms 

 possessing blood relations sufficiently intimate to enable 

 them to reproduce in kind through successive genera- 

 tions. We must not expect or look for absolute identity 

 among individuals of the same species or kind. We 

 should be satisfied with an agreement in the most es- 

 sential features. Then if the question arises concern- 

 ing the essential features of a character, it will have 

 to be said that the cssentialness of a character is to be 

 found in the constancy of its reappearance as successive 

 generations come forth. Upon such considerations is 

 a species based. 



But as has been suggested, species themselves change. 

 Within species we find varieties, which in time, we may 

 suggest, will continue to vary, for one or several of the 

 reasons stated, in a fixed direction until their essential 

 characteristics will be so diverse from the mother species 

 as to constitute a species themselves. There seems to 

 be a tendency in nature to encourage, as it were, this 

 divergence. If all forms of one species of insect were 

 identical, all would be common prey to the same ene- 

 mies. That is, if a certain parasitic insect or bird al- 

 ways attacked a certain species, these being all the 

 same, none would escape through the faulty recognition 

 of the enemy. But since this tendency to vary exists, 

 some escape by reason of dissimilar features, which 

 enables them, temporarily at least, to elude the recogni- 

 tion of the enemy. And so those escaping by reason 

 of these variations are consequently liable to reproduce 

 these variations in their offspring, until the variations 

 become fixed. Under such circumstances we consider 

 that a species has been developed. Individuals, then, 



