96 HEREDITARY CHARACTERS 



green plant. The worms are elongated and coloured green, 

 and at Roscoff live in the sandy tide-pools, fully exposed 

 to the sun's rays, looking like a mass of floating weed upon 

 the surface of the water. Now the stomach, and indeed the 

 whole of the alimentary apparatus, has disappeared in the 

 worm, and it lives exclusively upon carbohydrates formed 

 within its body by the Algae. The Alga has undergone most 

 profound changes. It has lost its membrane, thus allowing 

 its secretions and excretions to pass freely among the cells of 

 the worm, and it cannot live independently. If the worm 

 dies, the Algse contained in it die. All the allied forms of 

 this worm seek the dark, and live concealed under stones 

 and vegetation, and they are carnivorous. Convoluta, how- 

 ever, being dependent upon the well-being of the Alga for its 

 nourishment, seeks the sunlight and the surface of the 

 water. The sunlight is necessary to the growth and meta- 

 bolism in the Alga, which in its turn supplies food to the 

 worm which has ceased to be carnivorous. The two forms 

 are thus absolutely dependent upon each other and incapable 

 of living apart. 



All these cases involve changes in two different organisms, 

 and it is inconceivable that they can have been brought 

 about by the action of large mutations. The small variations 

 must have been selected by means of very gradual and very 

 small changes of environment. As far as the available evi- 

 dence goes at the present time, it would seem that a sudden 

 and great change in the environment tends to exterminate 

 the race subjected to it. Thus English dogs taken to India 

 do not thrive, and their offspring generally die out in one 

 or two generations. On the other hand, the dogs that have 

 been long subjected to the environment of India, thrive just 

 as well there as the English dog does at home. It is the 

 same with the human race. English children can be reared 

 in India only with difficulty, and in spite of our having 

 occupied the country to a considerable extent for many 

 years, the English race has never become established there. 

 Although the adult is capable of resisting environment, 



