The reality of the natural system is not afficted by the difficulty experienced by man in detecting it ; for it is no more to be expected that systematists should have already unravelled all the resem- blances between species contemplated by the Creator, than that anatomists should have arrived at the final cause of every organ of the human body.

The variety of classifications adopted by different naturalists, shows that we are still far from the true system of Nature, yet I think there can be no doubt that naturalists have already sketched out its principal features with considerable accuracy.

Who, for instance, can doubt that such groups as Vertebrata, Insecta, Mammalia, Pisces, Coleoptera, are not merely human generaliza- tions, but real apartments in the edifice of the Divine Archi- tect ? It is not, however, sufficient, that man should detect these natural groups, he must also give a definition of their characters, not of the superficial and arbitrary ones, but of the essential and important, and this is often the most difficult part of his task.

Although these essential charac- ters form the groundwork of the natural system, yet no rule can apparently be laid down for their determination in par- ticular cases.

All that man can do is to use his best judg- ment in selecting such characters for a group, as seem to him the most important in their influence on the vital functions of the beings which compose it.