LECTURE XVI. 447 



tween two extremes, ; so do we find, in the investigations of 

 the forms of the animal world and their development from the 

 egg, that here also the contrasts diminish, and that there 

 exist a number of forms which may well have been derived 

 from each other. Isidore Geoffrey Saint-Hilaire has pointed 

 out how the views of Buffon, as regards the limits and fixation 

 of the conception of species, gradually changed ; how, at first, 

 he ventured upon a hard definition which admitted of no 

 flexion, and how he gradually adapted it to the facts which he 

 had gathered through life, and which he was wise enough not to 

 reject because they clashed with his theory. I also lay claim to 

 the benefit derived from continued self-instruction, as regards 

 the change of my opinion, 



Darwin endeavours to show that every animal and every 

 plant sustains a constant struggle for existence, that it has to 

 contest for space, aliment and propagation, not only with the 

 surrounding physical agents, but with the whole organic 

 world, in which every other individual has the same rights to 

 space, aliment, and propagation. Every germ, every egg, 

 has a claim to life, but not every egg is developed, nor 

 every germ unfolded. Most succumb in this struggle, some 

 sooner, some later ; that individual only, which by itself or by 

 association is strong enough to issue from this battle as victor, 

 is enabled to live and to enjoy life. 



The question now arises, whether an adaptation of the indi- 

 vidual as well as of its progeny to the conditions of existence 

 can take place ? The question is, whether this adaptation by 

 continued improvement, continued breeding, if we may thus 

 express it, can lead to transformations which may compel us to 

 acknowledge these products as new types. On this point most 

 naturalists differ in opinion. 



The prevailing opinion, hitherto, was, that species are fixed 

 normal types, which may undergo changes within a very limited 

 sphere ; that they were the expression of a definite realised 

 idea ; that they were the separate unchangeable materials from 

 which, according to a creative plan, the structure of the organic 

 world had been erected. It was also asserted that species can 

 indeed perish, but cannot be transformed; that from time to 



