INTRODUCTION. 13 



There are two distinct varieties of these valuable aids, the nat- 

 ural and the artificial. The former is founded only on the charac- 

 ters that the advanced scientist uses in his classification. These are 

 often so exceedingly natural that to learn the proper position of the 

 object it must be destroyed. To learn something of an animal 

 through them the animal must be dissected, and the anatomy of its 

 nervous system, and the morphology of its osseous system, and the 

 structure of its heart be accurately observed, before it can be 

 classed. These arrangements may be, they often are exceedingly 

 valuable to the advanced scientist; to the amateur and the beginner 

 they are terrible. They haunt his dreams like horrid night-mares, 

 provided he is incautious enough, or ignorant enough to attempt to 

 use them-. I have not a word to say against such analytical tables. 

 I commend them for use in their proper places. They are important 

 for their purpose, but that purpose cannot be the beginner's or the 

 amateur's. It is the learned man that can delight his soul with such 

 a scientific key. To him it is instructive and helpful. To any one else 

 it is a bugbear and a horror. The beginner is not prepared to begin 

 with the nervous system of his "find," oreven with the structure of 

 its heart. He must have something nearer the surface and more easily 

 seen than are these parts. His key must deal with the external and 

 the evident characters. It is for him that the artificial key is in- 

 tended, and to him it is inspiring and helpful. 



This second kind of key, this artificial kind, is as its name indi- 

 cates, founded not on what the advanced investigator would use in his 

 classification of the animal or the plant, but on some obvious, prefer- 

 ably external, points that may be of no use in the art of classification, 

 but that can be used as crutches to help the student over the miry 

 and the stony ground to the hills of science where he will be able to 

 look back and smile at his former helplessness. These artificial 

 keys have this for their object, and only this. They aim to help over 

 the hard places; to encourage the user to go further, and to do more, 

 so that finally he may become an expert, when he will no longer 

 need any but the natural aids, or may even get along without any 

 other assistance than skilled eyes and a "learned touch." 



The beginner, however, must have a guide, especially if he be 

 trying to work alone. This is true in every department of natural 

 science. It is especially true in microscopy. The field is so im- 

 mense, the outlook so vast, the work so apparently endless, that with- 

 out an aid the beginner at once begins to flounder in his sea of 

 troubles, and before long he sinks discouraged to rise no more. He 



