io6 THE FIRST PRINCIPLES OF HEREDITY 



on the side exposed to the sun, but will produce rootlets on 

 that side if turned away from the light. Innumerable 

 adaptations of this kind exist among plants, known as 

 Heliotropism, Geotropism, etc. Animals, too, have this 

 power of adapting themselves to circumstances, as is 

 evidenced by the change of colour in certain caterpillars 

 in accordance with their surroundings, as shown by 

 E. B. Poulton. 



But — and this is the important point to grasp — all these 

 phenomena are nothing but reactions of the individual to 

 its surroundings, and do not concern his progeny. The 

 question in each case is : Are these adaptations inherited ? 

 do they form a factor in the production of the race ? 

 This is a different question altogether, which we are now 

 going to discuss. 



(b) Lamarckism versus Natural Selection. 



It is often assumed by laymen that evolution is possible 

 only on the assumption that the improvements undergone 

 by the parents are transmitted directly to the offspring, 

 thus leading to a gradual progress in ever-accumulating 

 ratio. This is the position taken up by Spencer, and was 

 the original theory of evolution propounded by Lamarck 

 himself. But though this school still survives, we must 

 not forget that Darwin, in his Origin of Species, formulated 

 a new theory of organic evolution, that of Natural Selection, 

 and it is this theory which is now generally accepted by 

 the scientific world. According to this theory organisms 

 change ; they vary, and those varying in a favourable 

 direction have the advantage over their fellows in the 

 struggle for existence, and survive, while the less favoured 

 individuals die out. As this process goes on unceasingly, 

 generation after generation, a gradual persistent improve- 

 ment of the race takes place. This theory explains organic 

 evolution without committing itself to the inheritance of 

 acquired characters, an assumption which, as we have seen. 



