CHAP. XII.] DIFFERENT KINDS OF CATS. 391 



proposers of new names. But there is yet unothcr difficulty. Though 

 the lion, tiger, leopard, &c., cannot bo mistaken one from another, 

 yet all lions are by no means alike, nor are all tigers or all leopards 

 alike. They all present individual variations, and these arc some- 

 times so marked that certain naturalists have thought it desirable 

 to distinguish one breed of lion by one name and another by another, 

 and so with leopards and other species. The questions then im- 

 mediately arise : (1) are these peculiar forms all "kinds" such as 

 we must take note of for our present purpose ? and (2) what are 

 the circumstances which should lead us to consider any given form 

 as constituting a distinct " kind " of animal ? 



Now, the various breeds of cats, such as we enumerated in the 

 first chapter, are called "varieties," while a lion and a tiger are not 

 called two "varieties " but two "species." What is the difference, 

 then, between a species and a variety ? 



§ 2. The exact jjhi/osojyiiicnl signification of the term "species" 

 will be considered in the last chapter ; here, we may take it to have 

 two meanings — one morphological ; the other, physiological. 

 According to the first of these, it signifies a group of animals which 

 are ahke in appearance. If two groups of animals differ markedly 

 in appearance, and if no transitional forms are known which bridge 

 over, as it were, the diff'erence thus existing between them, then 

 such two groups are reckoned as two distinct " species " according 

 to this first, or morphological, signification of that term, i.e., they are 

 morphological sj)ecies. The second use of the word " species " is to 

 denote a group of animals which can breed freely amongst them- 

 selves, but which, if united with animals of another appearance, will 

 not produce fertile cross-breeds with them ; that is to say, they will 

 not produce young which can go on indefinitely producing amongst 

 themselves a race of cross-breeds as freely as either set of parent 

 aninrals would have gone on reproducing forms like themselves. 

 Creatures which are in this way restricted, are pJ///siological species. 



As to the various breeds of domestic cats, we know that they can 

 be crossed and will produce perfectly fertile mongrels, and therefore 

 they are not physiologically " species," however truly each breed, 

 as long as it is uncrossed, will go on reproducing its own race — i.e., 

 will go on " breeding true." 



As to the wild cats of all kinds — lions, leopards, &c. — we know 

 that some of them will interbreed and produce young, but we have 

 no knowledge that such young vnll go on freely producing creatures 

 like themselves, while, from analogy with other animals, we should 

 be disposed to believe that they would not do so. Still we have aa 

 yet no observations to determine their specific distinctness physio- 

 logically, and therefore we must as yet be content to judge of them 

 morphologically, by the absence that is of intermediate forms 

 between the apparently distinct kinds. AVhenever new forms are 

 found so intermediate in character between two breeds previously 

 reckoned as distinct species, that these new forms quite bridge over 

 the diff'erence previously supposed to exist, then the supposed two 



