288 STUDY OF NATURAL HISTORY. 
instance, differing in some essential respects from 
that already before us; so that, from the various 
lights of different facts, concentrated in the point in 
question, we may form a correct judgment, whether 
the conclusion obtained from the first instance be a 
real principle of nature. If, for instance, any truth of 
anatomical science, collected from observation of our 
own species, were discerned also in the structure of 
most of the vertebrated animals, we should be almost 
sure that it was a general principle in that division of 
nature; but if, pursuing our examination into the 
invertebrated animals, we discovered the same prin- 
ciple under a different modification, and were en- 
abled to trace all the intermediate steps of gradation 
between the two extremes, we should then be sure 
that the principle was a general law of nature; since 
we found that it held also where the peculiar cir- 
cumstances, in which it was first observed, were 
wanting. 
(199.) The variety which is inbvodieail into any 
subjects by analogical argument, is in itself greatly 
serviceable to the business of instruction; it throws 
over the subject an inviting garb of attractiveness, 
thus alluring the attention of the general reader, and 
keeping alive the interest of the student. For ex- 
ample, in the analogy just quoted, what a pleasing 
and delightful illustration is given —by the circular 
progression of the seasons—of the circular pro- 
gression of beings in nature! both exhibited in 
friendly contrast with some of the greatest truths of 
the material and the spiritual world. How different 
are the analogical instances! and yet how harmoni- 
ous! The mind, thus led to the acknowledgment of 
