42 EVOLUTION OF LIVING OKGANISMS. 



vice versa, the seeds of the lowland form will grow into 

 the alpine form in the mountains. Moreover, if either 

 form be transplanted into the other region, it will soon 

 grow into the variety characteristic of its new habitat. 

 This change is accomplished by the new-growing tissues, 

 for the already formed tissues are no longer capable of 

 altering. Once fully differentiated they are " fixed." So 

 we see that organisms are moulded by their environ- 

 ment ; it is not the developed result which is transmitted, 

 it is not the modification which is inherited, but the 

 capacity for modification in certain directions, the 

 modifiability. 



The other example is taken from some experiments on 

 Primula sinensis, a well-known garden plant of which 

 there are several constant races or varieties with different 

 coloured flowers. If the variety with red flowers (P. s. 

 rubra) be grown in a hot-house at a temperature of be- 

 tween 15 and 20 centigrade, it will yield white flowers. 

 Brought back to a normal temperature it will again 

 bring forth red flowers. Which modification appears 

 depends on the stimulus. Now, there is another variety 

 (P. s. alba) which has white flowers at any temperature 

 from the normal to 20. At a temperature between 15 

 and 20 these two varieties would both bear white 

 flowers, yet they would not be the same ; they differ in 

 their hereditary factors, in their modifiability. In one 

 case the factors are such that they remain unresponsive 

 to change of temperature, in the other case they are 

 responsive. 



The argument may be briefly summarised as follows : 

 An organism is moulded as the result of two sets of 

 factors : the factors or stimuli which make up its en- 

 vironment, the conditions under which it grows up ; and 

 the factors of inheritance, the germinal constitution, 

 transmitted through its parent by means of the germ- 

 cells. No single part or character is completely "ac- 

 quired," or due to inheritance alone. Every character 



