32 GENETICS AND EUGENICS 



under identical conditions. Lamarckians assume that the 

 direct effects of the environment have accumulated and be- 

 come hereditary. Selectionists, on the other hand, maintain 

 that dwarf species were dwarfs originally and by nature, and 

 that they have found their way to the mountains because 

 they alone can survive under the harsh conditions there ob- 

 taining, whereas the more luxuriant forms were better 

 adapted to lowland conditions and have there crowded out 

 the dwarfs. It is evident that both explanations are logically 

 sound, though both cannot be true. Many experiments have 

 been tried to determine which best accords with fact, but the 

 results are not entirely conclusive because they are usually 

 capable of alternative interpretations, and each one inter- 

 prets them in accordance with the general theory which he 

 favors. A few typical experiments may be enumerated. 



(a) To altered salinity. Paul Bert, many years ago, at- 

 tempted to acclimatize some Daphniae (small fresh-water 

 Crustacea) to salt water by gradually adding salt to the 

 aquarium. At the end of forty-five days, when the water 

 contained 1.5 per cent of salt all the adults had died; but the 

 eggs in their brood-chambers survived, and the new genera- 

 tion arising from these flourished well in the salt medium. 

 This case has been cited as a case of inherited modification, 

 but such it clearly is not, because the parents did not succeed 

 in becoming acclimatized ; they died without becoming modi- 

 fied sufficiently to exist in the salt water. But their egg-cells 

 did become so modified, and the animals developing out of 

 them were acclimatized, through direct response to the en- 

 vironment, not through inheritance. 



Ferroniere transferred a worm (Tubifex) from fresh water 

 into sea water. The animal lived there and underwent cer- 

 tain changes of form (loss of bristles, etc.), which became 

 more deeply marked in later generations. After several 

 generations the animals were unable to live in the original 

 medium. This case is cited as showing inheritance of an 

 acquired modification. But it can with equal propriety be 

 interpreted as showing power of direct adaptation to changed 



