126 RECENT AD VANCES IN NA TURAL SCIENCE ix 



" permanent local variety," etc., into general use, and especially 

 by the wide differences of opinion as to the number or limits 

 of the species included in any given group of animals or plants 

 among naturalists who have made such group their special 

 study. 



2. Vast increase in the knowledge of the intimate structure 

 of organic bodies, both as revealed by ordinary dissection, and 

 by microscopic examination, a method of investigation only 

 brought to perfection in very recent years. By the knowledge 

 thus acquired has been demonstrated the unity of plan 

 pervading, under diverse modifications, the different members 

 of each natural group of organisms, at one time attributed to 

 " conformity to type," a so-called explanation which explained 

 nothing, but for which a vera causa may be found in de- 

 scent from a common ancestor. Wonderful gradations in the 

 perfection to which different structures have attained in the 

 progress of their adaptation to their respective purposes have 

 also been shown, and of still greater importance and interest, 

 the numerous cases of apparently useless or rudimentary 

 organs in both animals and plants, which were absolutely 

 unaccounted for under the older hypothesis. 



3. The comparatively new study of the geographical 

 distribution of living things, which has only become possible 

 since the prosecution of the systematic and scientific explora- 

 tions of the earth's surface, which have distinguished the 

 present century. The results of this branch of inquiry alone 

 have been sufficient to convince many naturalists of the 

 unsoundness of the old view of the distinct origin of species, 

 whether created each in the region of the globe to which it is 

 now confined, or, as many still imagine, all in one spot, from 

 which they have spread themselves unchanged in form, colour, 

 or other essential attributes to their present abodes, however 

 diverse in climate and other environments or conditions of 

 existence. 



4. Lastly, though most important of all, must be mentioned 

 the entirely new science of palaeontology, opening up worlds 

 of organic life before unknown, also showing infinite gradations 

 of structure, but mainly important as increasing our horizon 

 of observation to an extent not previously dreamt of in the 



