280 DEFINITE ACTION OF THE Chap. XXIIl 



in different men ; but in the tropics the effects of intemper- 

 ance differ from those caused in a cold climate ; ^^ and in this 

 case we see the definite influence of 0]3posite conditions. 

 The foregoing facts apparently give us as good an idea as 

 we are likely for a long time to obtain, how in many cases 

 external conditions act directl}^ though not definitely, in 

 causing modifications of structure. 



Summary. — There can be no doubt, from the facts given in 

 this chapter, that extremely slight changes in the conditions 

 of life sometimes, probably often, act in a definite manner 

 on our domesticated productions ; and, as the action of 

 changed conditions in causing indefinite variability is accumu- 

 lative, so it may be with their definite action. Hence 

 considerable and definite modifications of structure probably 

 follow from altered conditions acting during a long series of 

 generations. In some few instances a marked effect has been 

 produced quickly on all, or nearly all, the individuals which 

 have been exposed to a marked change of climate, food, or 

 other circumstance. This has occurred with EurojDcan men in 

 the United States, with European dogs in India, with horses 

 in the Falkland Islands, apparently with various animals at 

 Angora, with foreign oysters in the Mediterranean, and with 

 maize transported from one climate to another. We have 

 seen that the chemical compounds of some plants and the 

 state of their tissues are readily affected by changed condi- 

 tions. A relation apparently exists between certain characters 

 and certain conditions, so that if the latter be changed the 

 character is lost — as with the colours of flowers, the state of 

 some culinary plants, the fruit of the melon, the tail of 

 fat- tailed sheep, and the peculiar fleeces of other sheep. 



The production of galls, and the change of plumage in parrots 

 when fed on peculiar food or when inoculated by the poison 

 of a toad, prove to us what great and mysterious changes in 

 structure and colour, may be the definite result of chemical 

 changes in the nutrient fluids or tissues. 



We now almost certainly know that organic beings in a 



•' Mr. So(igwick, in ' British and Foreign Medico-Chirurg. Review,' Julv 

 1863, p. 175." 



