54 GENERAL PRINCIPLES OF ZOOLOOT. 



immediate infliienoe of changing environment, in part in the 

 varying nse and disuse of organs, brought about by alteraitions in 

 the conditions of life. Botli principles, they say, are sufficient, 

 even without the help of the struggle for existence, to explain the 

 phylogenesis of organisms. 



Influence of Environment. — To what extent can the environ- 

 ment bring about a permanent change in the structure of plants 

 and animals ? To decide this is no simple problem, on account of 

 the comjilexity of the factors entering into the question. 



In cases where the food-sujDply is altered, organisms change in 

 a very remarkable manner and within a short time; but these 

 changes (Nageli's ' Modifications through Nutrition') seem to have 

 no permanence. Plants which, found in nature in 2:ioor soil, are 

 transplanted into rich soil, or vice versa, soon acquire quite a 

 different appearance, and preserve this through the follo-\nng 

 generations, so long as they remain in the rich soil; but the plant 

 quickly returns to its former appearance when replaced in its 

 jirevious surroundi7igs. 



In general, a change seems to be the more permanent the more 

 slowly it has developed. In researches upon the influence of 

 environment, we can, therefore, rely soonest upon results if we 

 experiment with slowly-working factors, such as light and heat, 

 dry or moist air, different intensities of gravitation, of stimuli, 

 etc., which can be excluded from the environment of the organism. 



Use and Disuse. — Regarding the efficiency of use and disuse, 

 there is no doubt that the shape of an animal is influenced to a 

 great extent by the manner in which the organs are used. The 

 organs which are much used will become especially strong and rice 

 versa those which are not used will become weak. The onlv ques- 

 tion is whether these, in the strict sense of the word, newly-acquired 

 characteristics are transmitted to the offsjiring. or whether the 

 descendants, in order to attain to the same stage, must not repeat 

 in the same way use and disuse. In the latter case the cumulation 

 of characteristics, and with it the possil>ilitv that these may 

 become permanent, is excluded. It is to lie regretted that accurate 

 results are still lacking on a point so well adapted for experimental 

 treatment. At this time rudimentary organs strongly favor the 

 Laniarckian principle; for we see that cave animals, wliich for 

 many generations have lived iu darkness, are blind, either having 

 no eyes, or only vestiges of tlieui, incapable of function. This 

 seems to justify the view that this condition is attributable to lack 

 of use, since it ha,s lirought about a functional and anatomical 



