202 Mr. W. S. MacLeay on the Dying Struggle 



groups exist in nature but genera and species ; thus differing 

 toto ccelo from Dr. Fleming, who by advocating the binary 

 system goes to the other extreme, and states that there 

 is an infinite number of natural groups superior to genus 

 and species. The rule alluded to is, that groups of different 

 degree vary in their distance from each other. It is manifest, 

 for example, in the animal kingdom, that the distinction be- 

 tween two congeneric species does not depend on so important 

 a point of structure as the distinction between two contiguous 

 genera, nor this again on so important a point as the distinction 

 between families; and, to proceed in like manner on, the distinc- 

 tion between two sub-kingdoms like Vertebrata and Mollusca, 

 must be greater than all, because it depends on some most 

 important point of structure. A species is not only nearer to 

 its congener than Vertebrata are to Mollusca; but there exists, 

 although it may be impossible to calculate its exact value, a 

 manifest gradation of intervals between the various two con- 

 tiguous groups of the same rank, which are intermediate be- 

 tween a species and sub-kingdom. A vertebrated animal, for 

 instance, is marked out not merely by its vertebrated structure; 

 the very circumstance of not being vertebral argues other 

 most important distinctions in the general structure of the 

 animal. A gap therefore occurs between Vertebrata and all 

 the rest of the animal kingdom. Not only Lamarck but all 

 other naturalists have admitted it. How then is there no 

 continuity here ? I shall show that there is, and again I say 

 an hiatus is not a saltus. Continuity in gradation of structure 

 cannot exist, as we have seen, without intervals; and the size of 

 these intervals does not lessen the truth of the chain, because 

 some of the links may not yet be discovered. How then, you 

 ask, am I to prove that the chain is continuous? I answer, 

 Simply by ascertaining which animals of one group come the 

 nearest to those of the other. If there be no approximation, 

 if all the animals remain equally distant, then there is no con- 

 tinuity; but if one animal of the one group approaches to the 

 structure of the other, then there is a chain of continuity pos- 

 sessing indeed only one link, but not the less presenting a 

 mode of transition from one form to the other. Thus if the 

 only animal existing between Mammalia and Fishes were one 

 Penguin, it would still be in the path of passage. But if a 

 Tortoise existed in addition, the chain would be more complete, 

 and if one Frog existed also, the chain would scarcely escape 

 notice. In it there is a regular and obvious gradation of 

 structure, although the chasms remain vast. Nevertheless 

 there is no saltus or leap by nature over one form to another ; 

 and Linnaeus knew this, although by placing a whale imme- 

 diately 



