340 STUDIES, SCIENTIFIC AND SOCIAL chap. 



Species, however, which have a considerable range and 

 become dwarfed in some localities by adverse conditions 

 of drought, wind, or other causes, at once grow to a larger 

 size when cultivated and sheltered. Many of our British 

 botanists are now applying this test to distinguish those 

 forms of our native plants which owe their peculiarities to 

 germ-variation from those which have been modified 

 individually by the action of the environment. We are 

 often unable to decide by mere observation to which class 

 any particular variety or local form belongs, but cultivation 

 at once determines the point ; for while the former 

 transmit their peculiarities, how^ever minute these may be, 

 to their offspring, the latter revert at once to the parent 

 form. Mr. Beeby has proposed to call the former class 

 " intrinsic," the latter " extrinsic " varieties, useful terms 

 which indicate that the one are due to an internal cause, 

 are therefore stable, and show us the incipient stage of 

 species-formation ; while the other is merely an external 

 modification of the individual which has no stability, 

 being wholly due to the direct action of the environment. 

 It is very important to note the sharp distinction between 

 these two kinds of varieties, externally so alike, though 

 having a fundamentally different origin. There appears 

 to be no gradation from one to the other. The individually 

 acquired or extrinsic character, however long it may have 

 persisted, disappears instantly when the special environ- 

 ment that produced it is changed, as in the case of the 

 Texan Saturnia, the papery-leaved Arabis, and many 

 similar cases; while intrinsic characters — those due to 

 germ-variation — however slight they may be, as in the 

 various races of mankind, many of the closely allied 

 species of moths, and some of the sub-species or varieties 

 of our native plants, preserve their characteristic features 

 under greatly changed conditions. 



The cases now given of change in the individual due to 

 external causes and often of a very marked character, 

 render it exceedingly probable that a large portion of the 

 observable difference in the size of the jaws of civilised 

 man and of some domesticated animals, as well as all those 

 changes produced more or less suddenly by a change in 



