288 Scientific Intelligence. 
reply that, were 1 asked whether genera had a real existence in the ani- 
mate world, my answer would be that they undoubtedly have,—thou h 
not in the sense (which is so commonly supposed) of abrupt and discon- 
nected groups. I conceive them to be gradually formed nuclei, through 
the gathering together of creatures which more or less resemble each 
squadron have more or less in common. So that it is only in the middle 
points that these various groups, respectively, attain their maximum,— 
every one of which (by way of illustration) may be described as a con- 
centric bulb, which becomes denser, as it were, in its successive compo- 
nent layers, and more typical, as it approaches its core.” 
The main topic of the work is the variations which species undergo. 
He illustrates it by facts and urges the importance of its study as the 
foundation of our knowledge of species. With every species in nature, 
organic or inorganic, there appears to be a normal type admitting of 
librations in many of its characters, on either side through external influ- 
ences; and the complete idea involves a knowledge of the extent and 
simply absurd; for as Lord Bacon has remarked, the “subtility of Na- 
ture far exceeds the subtility of reasoning :” but if, by a careful collation 
of facts, and the sifting of minute particulars gathered from without, 
we must except those races of animals and plants” 
ard of evidence as we require for those which have remainet 
for ever untouched and free,—especially so, since (as we have already 
