400 



ORGANIC GROWTH 



SEC. 



explain the recrescence of complex tissues and organs in the 

 higher animals, it is necessary to assume, not merely that the 

 parts have a general relationsliip to one another, a general 

 inherited formative tendency, but that they have at the same 

 time a special formative tendency directed towards the 

 structure of tlie part to which they belong. This special 

 relationship shows itself in the fact that these parts are 

 specially correlated with one another which are derived from 

 the same germ-layer, epidermis, hair and horns, for instance, 

 in Mammalia. 



Tlie higher the ^rade of oroianisation in the animal kino- 

 dom, the greater the division of labour, the more complex the 

 organs, so much the less are all cells in a condition to repro- 

 duce different parts or the whole body, so much the more are 

 their powers restricted to the redevelopment of particular parts. 



Thus it is evident that the cells of certain reoions of the 

 body of the higher organisms are endowed by heredit}^ with 

 particular tendencies of growth : they have a tendency by 

 union with one another to constitute, to complete, particular 

 parts. This is an instance of the establishment by functional 

 action and heredity of definitely directed forces. 



This is shown to me in the simplest way by another ex- 

 periment of Yochting's : he hung up a twig of willow in a posi- 

 tion the reverse of the natural, in a vessel where it was 

 exposed to moisture and the other conditions favourable to 

 growth. This twig produced roots not only at the lower end, 

 where their development was favoured by gravity, but also 

 and in superior numbers at the upper end — evidently there in 

 consequence of the established hereditary tendencies of growth. 



Subsequently Herr Vochting drew my attention to some 

 remarks of his which well express the significance of the 

 examples I have previously given of definite growth- 

 tendencies due to acquirement and inheritance, although for 

 his part he puts forward the view I maintain as only 



