384 THE EVOLUTION THEORY 



development, and give rise under normal external influences to normal 

 parts, while under unusual influences, if these are not such as to 

 prohibit development altogether, they may give rise to an abnormally 

 formed part. It must not be forgotten that most composite parts — 

 indeed, strictly speaking, all the parts — of an animal are not controlled 

 by a single determinant, but by the many which successively determine 

 the character of the cells and define the path of development of the 

 part in question. There are no determinants of ' characters,' but only 

 of parts ; the germ-plasm no more contains the determinants of a 

 ' crooked nose ' than it does those of a butterfly's tailed wing, but 

 it contains a number of determinants which so control the whole 

 cell-group in all its successive stages, leading on to the development 

 of the nose, that ultimately the crooked nose must result, just as the 

 butterfly's wing with all its veins, membranes, tracheae, glandular 

 cells, scales, pigment deposits, and pointed tail arises through the 

 successive interposition of numerous determinants in the course of 

 cell-multiplication. 



But in both processes we must presuppose normal conditions of 

 development. In regard to the butterfly we know that abnormal 

 conditions, such as cold during the pupal period, can cause considerable 

 variation in the colour and marking of the wing, and in regard to the 

 nose it can scarcely be doubted that, for instance, persistent pressure 

 on the nasal region would result in a considerable deviation from the 

 hereditary form. 



The case of the lithium-larvae is similar. Here the chemical 

 conditions of the first segmentation-cells are modified by the presence 

 of the lithium -salts, and the determinants which make their way out 

 of the nucleus in the first and in subsequent cell-generations find an 

 unusual soil for their activity, which diverges further and further 

 from the normal with each successive cell -generation. Thus the 

 whole animal is abnormally formed. The process may perhaps be 

 compared to a plant which is negatively geotropic and positively 

 heliotropic, that is, the stem of which tends to grow straight upwards, 

 while all its green parts grow towards the light. If a plant of this 

 kind have light shed on it from one side only, the stem with its 

 leaves will grow obliquely towards that side. If the plant be then 

 turned round so that it receives light from the other side, the stem in 

 its further growth will curve in a direction opposite to that which 

 it took before, and so by continually changing the position of the 

 plant in relation to the light one could — theoretically at least — produce 

 a plant with a zigzag stem. But this would not furnish any evidence 

 against the presence of determinants ; there are no ' upright deter- 



