l8 THEORY OF ORGANIC EVOLUTION. 



less virility and is avoided by nature, this is the 

 result of injurious influences which make themselves 

 felt later on. This is because incompatibilities may 

 be present in too closely related idioplasms and 

 these are sources of weakness in unrestricted develop- 

 ment. The more complicated is the idioplasm, the 

 oftener this occurs, whereas absolute lack of cross- 

 ing is not detrimental to the simplest (asexual) 

 organisms. 



II. ACTION OF EXTERNAL INFLUENCES. * 



The environment provides the organism above 

 all with force and matter for its life processes. It 

 causes no permanent variation and has only an 

 ontogenetic significance, if the limits of the idio- 

 plasmic elasticity are not exceeded ; it maintains the 

 growth and metabolic assimilation of the individual, 

 and conditions individual (not hereditary) differ- 

 ences, which constitute "nutrition varieties. " (See 



* In order to explain adaptations Nageli assumes that external influ- 

 ences, if acting at the same point in a given manner for a long time, may 

 induce slight adaptive variations which are perpetuated and increased. 

 On the important subject of adaptation in general Nageli is almost diametri- 

 cally opposed to Darwin and Weismann. Nageli assigns to the principle ot 

 utility a very limited sphere; Weismann regards adaptation as all-powerful. 

 According to Nageli, the organic world would have become much what it is, 

 if natural selection and adaptation had performed no part in the operations 

 of nature. He aptly says, that natural selection prunes the phylogenetic 

 tree, but does not cause new branches to grow. He allows that the prin- 

 ciple of selection is well suited to explain the adaptation of organisms to 

 their environment and the suitableness and physiological peculiarities ot 

 their structure, but he asserts that in the definiteness of variation of plant* 

 and in their progressive differentiation there is evidence of a higher and 

 controlling perfecting principle. Tram. 



