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those which existed formerly, but which have utterly disappeared 

 from the earth. Go back in imagination to those remote ages, in 

 which life would seem to have first dawned upon its surface, and 

 note the ever progressing evolution that has been going on from 

 that time to this. Witness the graduated succession of allied 

 forms, plants aswell^as animals — altered life keeping step with 

 altered physical conditions of the earth — a succession not forming 

 a single series only but thousands of other series branching off in 

 all directions, ramification upon ramification, becoming in the end 

 a vast conglomerate of affinities, each form showing more or less 

 relationship to all the rest ; life at the same time seen continually 

 rising in the scale of organisation and assuming a more and more 

 complex structure, until it culminates in man, the lord of all. 



From this view of life, taken in all its generality, we are led to 

 the apprehension of a significant truth lying at the bottom of all 

 our sciences both physical and natural, and greatly enhancing 

 their importance and the interest that attaches to them, viz., that 

 the things which these sciences take knowledge of are not things 

 ever continuing the same with fixed distinguishing characters, but 

 things on the contrary ever undergoing slow but sure change. 

 There is nothing in nature absolutely at rest. Not only is the 

 face of the earth, its hills and valleys, its rivers and seas, silently 

 altering from day to day — every change bringing with it a 

 corresponding change in the relative distribution of plants and 

 animals — but there is not a single stone, in which there is not 

 going on perpetually a redistribution of its molecules under the 

 influence of the same forces which, so far as we see, regulate the 

 movements of the whole universe. In like manner, there is 

 probably no one plant or animal, whether viewed in the species or 

 the individual, exactly what it was at some former pei'iod of 

 its history. Its environments are always changing ; and to these 

 changes its organisation is always seeking to adapt itself, so that 

 there may be harmony between the two, between what is within 

 and what is without, its whole system undergoing for the purpose 

 slight modifications, more or less permanent for a time, but in the 

 end giving way to others, and according to their nature and 

 importance extending their influence to the next generation. 



