406 THE STUDY OF ANIMAL LIFE CHAP. 



are more or less active agents, and changes in environment 

 offer opportunities for effort, e.g. for trying to find a new 

 haunt. 



It has often been pointed out that an individually 

 acquired modification may serve as a life-saving screen 

 until an innate variation with similar result has time to 

 establish itself. Thus artificial immunity may be a 

 useful temporary modification until natural immunity 

 arrives as a germinal variation if it does so arrive. 



In the case of mammals the unborn offspring may be 

 seriously handicapped by the ill-nourished, over-strained, 

 or poisoned state of the maternal body. There is no 

 transmission of acquired characters in the technical sense, 

 but there is ante-natal deterioration and arrestment of 

 the offspring as the result of abnormal nurture on the 

 parent's part. Some evidence exists which goes to show 

 that deeply saturating parental modifications, such as 

 the results of poisoning, may affect the germ-cells them- 

 selves. The influence very probably affects the cytoplasm, 

 rather than the chromosomes. 



There is little likelihood that we are at an end of the 

 question as to the influences of modifications (nurture- 

 effects) on inheritance, and a useful hint of the subtlety 

 of the problem may be got from a brief consideration of 

 the most important British investigation on the subject- 

 Dr. Agar's study of a water-flea (Simocephalus) a little 

 crustacean with two valves. Under certain nutritive 

 conditions the crustaceans acquired a peculiar reversal 

 of their shell-valves, doubtless as the result of altered 

 metabolism. After the eggs had appeared and grown in 

 the ovary the animals were restored to normal conditions. 

 In due time the eggs developed into forms with reflexed 

 shell- valves such as their parents had acquired. Later 

 on, however, when the parents laid again, the abnormal 

 effect was seen only to a very slight degree-, and in a 

 third brood it had dwindled away. The probability is 

 that the abnormal nurture resulted not in any disturbance 

 of the inheritance, but in the formation of some peculiar 

 non-living metabolic product, which was included in the 

 cytoplasm of the egg, passed passively into the body 



