WHAT IS A SPECIES ? 131 



(7) possibly other unknown causes, e.g., it isdifficultin the present 

 state of our knowledge to account for the prevalence of a blue 

 sheen in the dominant Papilionina of the Assam subregion. 



(8) Those belonging to (1) are the newest and most dominant 

 types. Those belonging to (2) and (3) are older, while class (4) 

 are very ancient, often not merely as specific forms but belonging 

 to generically ancient types of structure. 



(9) There exists among all creatures a progressive or rather a con- 

 servative tendency towards fixity of type. 



(10) Almost all creatures around us now are admirably adapted to 

 their place in nature — observations of so-called evolution in the 

 making being extremely rare. 



(11) By consequence any tendency to vary would, ceteris paribus, 

 be contrary to nature's great object — the preservation of the type. 



(12) Therefore unless the environment is changed, there is no a 

 priori likelihood that any variation will occur. 



(13) A change of environment occurs either by the type itself 

 migrating to a new locality or by a new environment coming to 

 the type through geological upheaval causing either a complete 

 change of climate or by uniting the region with another not 

 previously connected letting in a crowd of forms whose presence 

 entirely alters the aspect of the struggle for existence, or both. 



(14c) Species which seek a change of environment themselves will 

 belong mainly to (1). Although the laws of migrations are 

 imperfectly known, it may be said generally that they probably 

 arise originally from pressure of numbers and, that their direction 

 is determined in the first instance, at least among winged 

 creatures, by the winds and that types once moved in this way 

 usually acquire for a time a regular migratory habit. (This 

 ignores all forms spread artificially by man and seasonable 

 migrations of birds.) 



(15) In the event of the second alternative the older species will 

 many of them have become so fixed that they will not be able to 

 adapt themselves to the new conditions and will disappear or 

 become rare. Others will have to change and change with con- 

 siderable rapidity until they acquire a form and constitution suited 

 to survive, and most of the intermediate forms will have little 

 chance of perpetuating themselves. 



