( cxxxvi ) 



distribution. The preponderant opinion appears to be that most 

 of our species or their near ancestors came from tropical or sub- 

 tropical regions or descended to us from periods when similar 

 high temperatures prevailed, rendering production more rapid 

 and activity greater, and the opinion is supported by the 

 abundant evidence we have in England of repeated invasions 

 by overflow from the continent of Europe, successful for a 

 brief time, during which climatic conditions approach more 

 nearly to those of a warmer country. 



We find that, as a rule, a species has its metropolitan 

 centres — it may be covering, continuously or discontinuously, 

 a large geographical region, — ^from which centres it thins off 

 in various directions. To take a familiar example, we see a 

 very large number of species abundant in many of the various 

 parts of Europe become scarcer as we proceed northwards, for 

 example within our islands, in the northern parts of which 

 they die out. A smaller number have their metropolis in the 

 north, and become rarer as we proceed southwards till they 

 disappear. When plants thin off in this way we attribute the 

 reduction to the direct effect of climate, and this doubtless has 

 its effect on insects. But in their case it is much more limited 

 in its operation. Insects can move and in various ways 

 protect themselves, and therefore the element of their personal 

 habits comes in, and the efficiency of these depends on health 

 and vigour. I suggest that, apart fi-om the fact, referred to 

 later, that a cold climate will not admit of such a wealth of 

 organisms as a warm climate will, the thinning off I have 

 referred to may indicate, not so much the direct effect of the 

 climate and other surroiindings in killing off, as their operation 

 in diminishing health and vigour, and in this manner affecting 

 the activities of insects. Many of these activities are of 

 course connected with the pairing instincts, such as the 

 coiurtship of butterflies and grasshoppers, or with the pro- 

 curing of food. But, as stated, many cannot be brought under 

 either of these heads. 



Motive sjyrings of activity — Emotions. 



As to the varied activities of insects, I think we cannot 

 properly appreciate what they may mean without referring to 



