440 BIOLOGY IN RELATION TO OTHER NATURAL SCIENCES. 



from moment to moment the organism governs itself. And the \Yhole 

 process is a necessary outcome of the fact that those organisms are 

 most prosperous which look best after their own welfare. As in that 

 part of l)iology which deals with the internal relations of the organism, 

 the interest of the individual is in like manner the sole motive by which 

 every energy is guided. We may take what Treviranus called selfish 

 adaptation — Zweclmassigleit fur sich selher — as a connecting link 

 between the two branches of biological study. Out of this relation 

 springs another which J need not say was not recognized until after 

 the Darwinian epoch — that, 1 mean, which subsists between the two 

 evolutions, that of the race and that of the individual. Treviranus, no 

 less distinctly than his great contemporary Lamarck, was well aware 

 that the affinities of plants and animals must be estimated accordnig 

 to their developmental value, and consequently that classitication must 

 be founded on development; but it occurred to no one what the real 

 link was between descent and development; nor was it indeed until 

 several years after the publication of the "Origin" that Haeckel enun- 

 ciated that "biogenetic law," according to which the development of 

 any individual organism is but a memory, a recai^itulation by the indi- 

 vidual of the development of the race — of the process for which Fritz 

 Miiller had coined the excellent word "phylogenesis;" and that each 

 stage of the former is but a transitory re-appearance of a bygone epoch 

 in its ancestral history. If therefore we are right in regarding onto- 

 genesis as dependent on i:>hylogenesis, the origin of the former must 

 correspond with that of the latter; that is, on the power which the race 

 or the organism at every stage of its existence possesses of profiting by 

 every condition or circumstance for its own advancement. • 



From the short summary of the connection between different parts 

 of our science you will see that biology naturally falls into three divi- 

 sions, and these are even more sharply distinguished by their methods 

 than by their subjects, waiweXy^ physiology, of \>'\i\ch the methods are 

 entirely experimental; morphology, the science which deals with the 

 forms and structure of plants and animals, and of which it maybe said 

 that the body is anatomy, the soul, development; and finally, occology, 

 which n^es all the knowledge it can obtain from the other two, but 

 chu'tiy rests on the exploration of the endless varied phenomena of ani- 

 mal and plant life as they manifest themselves under natural conditions. 

 This last branch of biology — the science which concerns itself with the 

 external lelations of plants and animals to each other, and to the past 

 and ijresent conditions of their existence — is by far the most attractive. 

 In it those qualities of mind which especially distinguish the naturalist 

 find their highest exercise, and it represents more than anj' other branch 

 of the subject what Treviranus tenned the "philosophy of living nature." 

 Notwithstanding the very general interest which several of its problems 

 excite at the inesent moment I do not propose to discuss any of them, 

 but rather to limit myself to the humbler task of showing that the fun- 



