xviii INTRODUCTORY LETTER TO SENATOR MOSSO 



attempt to learn something as to the essential character 

 and the cause of that loss. 



Age causes many progressive changes in the or- 

 ganism, but none which are more obvious and more 

 accessible to exact study than those of growth. Thus 

 I was led to make my first experiments on growth. 

 It soon appeared that the scope of the inquiry was 

 expanding, and it has not been until now that the 

 matters included have become sufficiently co-ordinated 

 to justify their collective publication, and yet the 

 research remains fragmentary, narrow, and incom- 

 plete. I can make no pretence of having solved the 

 manifold problems of senescence, but I hope that you 

 will at least find some of them more clearly formulated 

 than hitherto, and also some real additions to our 

 positive knowledge. 



For the purpose of studying growth as a function 

 of age it was desirable to eliminate the influence of 

 external conditions of a variable character as far as 

 possible ; the readiest way to accomplish this was to 

 choose a self-regulative organism ; accordingly one of 

 the higher vertebrates was considered preferable, be- 

 cause of all organisms they are the most independent 

 of outside circumstances. It remained only to pick 

 out a convenient species ; various considerations led 

 to the choice of the guinea-pigs, Cavia cobaya. This 

 animal offers the following advantages : it bears con- 

 finement well, is robust and but little liable to disease, 

 breeds readily, is easily managed and fed, and gentle 

 when handled ; its maintenance is much less costly 



