CONCEPTIONS OF FORM. 503 



though even here we must be satisfied with following the causes and eflfects of growth 

 in their general outlines only. Even here also it must be left for the future to analyse 

 the processes further, and to resolve the causal relations into their several factors. 



A better survey of the phenomena lying before us will be obtained if we first 

 distinguish between the internal and external causes of growth. We regard it as the 

 effect of internal causes of growth when by means of the growth of one organ the 

 growth of another organ of the same plant is promoted or hindered, thus recognising 

 a mutual causal relation in the growth of different organs of a plant. This mutual 

 causal relation has been termed the correlation of the growth of the organs of a plant. 

 On the other hand, however, experience also shows that the growth of individual 

 organs, or parts of organs, may be promoted or hindered by purely incidental 

 external actions, by light, gravitation, contact, pressure, and other influences of the 

 environment. 



Here, however, we enter upon a province where it is very difficult to obtain 

 certain information, not only because the connection between causes and effects 

 is in itself extremely complicated in each of the two cases, but still more 

 because experience teaches that the relations of growth of the plant to the external 

 world, as well as the correlations, are variable in the highest degree. Processes 

 of growth which in the one species of plant are established once for all, and appear 

 spontaneously by the internal connection of development, are only called forth in 

 other species by definite external influences, and no sharp line is in this respect to be 

 found anywhere. We are in the habit of saying that such phenomena are in the one 

 case hereditary and constant, while in the other case they are caused by irritability; 

 but it must not be forgotten that nothing is explained by such statements, only a 

 logical classification — i.e. one suited to our preconceived ideas— is obtained. 



Here also I regard it as beyond my task to state all that is known with 

 respect to correlations and external conditions of growth ; but rather, in this and the 

 following lectures, to make clearer, with a few examples only, the meaning of the 

 questions here raised, and to show with what problems the theory of growth has 

 to concern itself, and at the same time I must refer to the fact that it is exactly in the 

 post-embryonic conditions of growth, with which we have chiefly to do here, that the 

 organs of the plant are in a high degree sensitive to internal as well as to external 

 influences. In spite of all difficulties and indefiniteness, however, profound interest 

 is always attached to such considerations, because the question which here presents 

 itself is as to what causes determine the internal structure and the external form of 

 organisms. Only a short time ago the forms or organic bodies were regarded as 

 something lying beyond the region of causality, and every organism was held to be a 

 more or less successful realisation of an ' idea.' We here stand therefore on the 

 boundary between two different modes of viewing the universe, of which the one, the 

 idealistic — or, to put it in a more concrete form, the platonic — knows and will know 

 nothing whatever of effective causes in the domain of organic form; whereas my 

 view starts from the fundamental notion that organic forms, exactly as the configura- 

 tions of crystals, and all matters of form whatever — it matters not whether we are 

 concerned with the shape of a drop of water, a planet, a cloud, or any other product 

 of nature — must be called forth by effective causes, which are determined by the 

 nature of matter and its forces under the given circumstances. 



