PREFACE. 



Biology is the science wliich treats of all living 

 organisms, from Man to the lowest plant. Ko natm*al 

 science is at present more keenly pursued or with more 

 effect. The advances of Astronomy and Geology have 

 produced great changes in men's minds during the last 

 three centuries : Biology is producing changes at least as 

 great, in the present age. So rapid has been its progress 

 that the Natural History of Animals and Plants needs to be 

 rewritten — the field of Nature being surveyed from a new 

 stand-point. Such a history may be written in two ways : 

 (1) Living beings may be treated as one whole, their 

 various powers and the more general facts as to their 

 organization being successively portrayed as they exist 

 in the whole series ; or (2) one animal (or plant) may be 

 selected as a type and treated of in detail, other types, 

 successively more _ divergent in structure from the first, 

 being described afterwards. 



In following the latter mode, we may either begin with 

 one of the most simply organised of living creatures and 

 gradually ascend to the highest and most complex in 

 structure; or we may commence with the latter, and 

 thence descend to the consideration of the lowest kinds of 

 animated beings. 



Historically, it is the latter course which has been 



