Development. 69 



of the forms from the simplest to the most complicated. This 

 leads to the recognition of the general plans upon which the 

 whole phenomena march, and enahles us to comprehend, when 

 thoroughly studied, the difference and apparent discrepancies 

 which are constantly observed in comparing isolated obsci'- 

 vations. 



Two methods exist by which these laws of variation and com- 

 bination may be studied, both of which are used by the physio- 

 logist, each assisting to clear up the difficulties of the other, 

 and, as it were, demonstrating its problems by a different pro- 

 cess — these are, Cowparative Phjjsiolof/y and the study of Deve- 

 lopment. In the latter we pursue the unfolding of the individual 

 body from the earliest and simplest recognizable stage, when it 

 is a simple microscopic germ ; in the former we are enabled to 

 detect an analogous (but by no means similar) series of forms, 

 with variations on different types, in the countless varieties of 

 individual kinds which lie between the microscopic infusorium 

 and man, or between the yeast-cell and the forest-tree. 



The study of the development or unfolding of structures, con- 

 stitutes the only safe foundation for a knowledge of the pheno- 

 mena of life, both in the animal and vegetable kingdoms ; but it 

 stands on relatively higher ground in tlie latter, from the circum- 

 stance that vegetable life may be said to consist wholly of deve- 

 lopment. Almost all change here consists of the production of 

 new structures or tlie completion of older, without anything 

 which. can be compared with the reparation or renewal of struc- 

 tures occurring in tlie nutrition of animals. Man and the higher 

 animals exhibit unmistakeable examples of the effects of the 

 nutrition of which we speak. They attain at a certain period 

 their full growth, and, remaining for a siiorter or longer period 

 \n i\\e\v prime, \\\ex\. begin to descend into decay. Tliroughout 

 all this period two processes run side by side, — development of 

 new structure and absorption of part of that previously formed ; 

 tliroughout the first part of the life development exceeds absorp- 

 tion, in the latter part absorption exceeds development ; but both 

 are in more or less active operation throughout life. In plants 

 there is no analogous set of circumstances ; there exists no similar 

 process of absorption of used-up parts. The life, excej)ting of 

 course during the hybernating periods, when all the processes are 

 at rest, as during the winter in our latitudes — the life consists of 

 constant new formation of substance, and the form of the entire 

 individual is undergoing constant change. In annual plants we 

 cannot say that the nidividual is in its prime at any ejioch, cer- 

 tainly not at the period when it is usually most attractive — that 

 of flowering, and hardly at tlie tim(> when the Iruit is ripe, since 

 then the great body of the structure is on the verge of dissohition. 



