CHAP. XX. IN THE ANIMAL AND VEGETABLE WOKLD. 395 



of a large and conspicuous animal or plant, and as to the 

 smaller kinds, many of them may be conceived to have stolen 

 in unseen, and to have spread gradually over a wide area, like 

 species migrating into new provinces.* 



It may now be useful to offer some remarks on the very 

 different reception which the twin branches of Lamarck's 

 development theory, namely, progression and transmutation, 

 have met with, and to enquire into the causes of the popu 

 larity of the one, and the great unpopularity of the other. 

 We usually test the value of a scientific hypothesis by the 

 number and variety of the phenomena of which it offers a 

 fair or plausible explanation. If transmutation, when thus 

 tested, has decidedly the advantage over progression, and yet 

 is comparatively in disfavour, we may reasonably suspect that 

 its reception is retarded, not so much by its own inherent de 

 merits, as by some apprehended consequences which it is 

 supposed to involve, and which run counter to our precon 

 ceived opinions. 



Theory of Progression. 



In treating of this question, I shall begin with the doctrine 

 of progression, a concise statement of which, so far as it relates 

 to the animal kingdom, was thus given twelve years ago by 

 Professor Sedgwick, in the preface to his Discourse on the 

 Studies of the University of Cambridge. 



( There are traces,' he says, 'among the old deposits of the 

 earth of an organic progression amdng the successive forms of 

 life. They are to be seen in the absence of mammalia in the 

 older, and their very rare appearance in the newer secondary 

 groups; in the diffusion of warm-blooded quadrupeds (fre 

 quently of unknown genera) in the older tertiary system, 

 and in their great abundance (and frequently of known 



* Principles of Geology, 1st ed. 1832, vol. ii. ch. xi. ; and 9th ed. p. 706. 



