CHAPTER II 



THE METHOD OF DEVELOPMENT 1 



The adaptation of the individual The method of adaptation Variations 

 Modifications Progression Retrogression The recapitulation of the life- 

 history The opinions of embryologists. 



I 



38. fNDIVIDUALS \vho survive and have offspring, especially 

 if they have a full quota of offspring, must necessarily 

 have been well fitted from first to last, from ovum to adult, 

 to their surroundings. The changes in the individual during 

 development, especially in the higher animals, are very great, but, 

 speaking generally, they are always adaptive. Thus the larva of 

 the dragon-fly which inhabits the water is structurally fitted for a life 

 in it, and undergoes an adaptive change when, as perfect insect, 

 it migrates to land. Thus again the human embryo in the womb 

 is fitted to an environment enormously different to that in which 

 life is possible to the adult. All, or almost all, the changes it 

 undergoes during development are adaptive also. Offspring, to 

 survive in the same environments, must closely resemble their 

 parents. This necessary resemblance is secured by development 

 along similar lines. Step by step, from germ to adult, the child 

 treads in the developmental footsteps of the parent. When, as 

 occasionally happens, the recapitulation of the parental develop- 

 ment is not fairly close, the child is a * monster,' out of harmony 

 with its environment, and so inevitably perishes. 



39. Though every normal child resembles its parent as a whole, 

 it invariably differs in details, which, though innumerable, are 

 minute as compared to the likenesses. Thus, though the normal 

 offspring of a human being is always another human being, it is 

 never an exactly similar one. These differences, when innate 

 (i.e. germinal) are known as variations, and must be carefully dis- 

 tinguished from differences which result merely from an unequal 

 play of stimuli (nutrition, use, injury, etc.) on parent and child. 

 The latter are acquired differences or modifications, terms which, 

 as we have seen, are also used, though incorrectly, to distinguish 

 1 See Appendix, i-io. 



