36 INTRODUCTION. 



(23.) To those who are collecting facts for their 

 own use, and not merely to supply others with ma- 

 terials for generalization, it may be useful to be 

 taught the importance of observing a particular fact 

 in a sufficient number of instances, before coming to 

 any conclusion respecting it. Long and repeated 

 observation is required in most cases to enable the 

 mind to generalize correctly. And this, which is 

 true in all sciences, is especially so in the science of 

 Natural History, in which there are as yet so few 

 principles well founded and universally received. 

 In proportion to the advance of our knowledge of 

 such principles, will be our safety in taking analogy 

 for our guide. Yet analogy is rarely to be trusted, 

 as those will testify, who have studied longest the 

 book of nature, and searched it the most deeply. 

 We often find quite a different law (if we may use 

 the term) prevailing in two cases which at first view 

 appeared parallel. By law we mean some rule 

 which seems to regulate either the structure or the 

 habits of animals. Let us take the instance of 

 aquatic and land animals. What observer who was 

 acquainted with the mode of respiration in the land 

 mammals, and then found fish breathing by gills, 

 would at first suspect that the cetaceans accorded 

 with the former in the organs adapted to the ex- 

 ercise of this function. And in the case of certain 

 groups lower down in the system we may observe 

 the converse of this singular phenomenon. The 

 land crabs breathe as exclusively by gills, as any of 

 their congeners which reside always in the water. So 

 also, it is now found, do the common wood-lice, 



