MODIFICATION OF REPRODUCTIVE PROCESS 67 



cell to live in this medium just as she has adapted the 

 inhabitant of the pool or pond to its own watery medium. 

 This internal medium of the animal body however is 

 not pure water but a very complicated mixture. The 

 incessant living activity of the protoplasm causes constant 

 formation of waste materials, and these are discharged 

 into the fluid. Now these waste products are by no 

 means the same : they vary in different kinds of cells 

 and in different organs each of which contributes its 

 quota to the mixture. If any particular organ omits 

 its contribution, or if its contribution is abnormal in 

 quantity or quality, then the composition of the internal 

 medium is altered and the health of the whole body is 

 liable to suffer, owing to the fact that its cells are 

 adapted to live in the medium of a certain ' normal ' 

 composition. 



One of the chief obstacles w r hich have lain in the way 

 of animals becoming adapted to life on land has had 

 to do with the early stages in their life-history. For 

 these early stages repeat in a somewhat crude form 

 earlier stages of evolution, in which the habit was purely 

 aquatic. Thus if one examines an early stage in the 

 development of one of the higher Vertebrates say a 

 fowl, or a human being one finds along the sides of 

 its neck gill-openings such as those of a fish, although the 

 fowl or man will never have occasion to use them for 

 breathing ; again the main blood-vessels are arranged 

 on the same plan as those of a Fish for the conveying of 

 blood to and from the gills ; again the skeleton is of a 

 simple gristly nature like that of one of the lower fishes ; 

 and so on with various other organs of the body. Any 

 zoologist finding such a creature living free in Nature 

 instead of within the egg-shell or the body of the parent 



