28 WHAT IS A SPECIES? 



It seems to me that varieties of living beings are 

 like shades of color — indefinite in number, and blend- 

 ing into one another like the tints of the rainbow. 

 Though there are shades in the middle of the spectrum 

 which one might call green, and another might call 

 blue, still, blue is blue, and green is green, and ever 

 remain so. We may give names to the intermediate 

 colors, like light blue or greenish blue, but no one 

 doubts that each of these shades is distinct, though 

 not easily defined. So, though we doubtless must 

 reduce many of our present species to the rank of 

 varieties, and though there may be many varieties 

 concerning which there will be differing opinions, 

 and which will be variously classed, even by thought- 

 ful observers — still, I believe the great forms of life, 

 the true species, exist unchanged, and can vary only 

 within fixed and narrow limits, unless it be by the 

 express act of creative power. 



Next on our list comes a small genus of mollusks, 

 which are almost exclusively confined to the west 

 coast of America. A good representation of one of 

 these shells is shown in Fig. 9, which illustrates the 

 common Unicorn shell, Monoceros lapilloides, Conr., 

 Mo-nos'-e-ros lap-il-loi'-des. The Greek name, Mo- 

 noceros, is exactly translated by our word 

 "Unicorn," which is derived from the 

 Latin, and both of them mean single-horn. 

 The specific name, lapilloides, means, re- 

 sembling a pebble. The little horn near 

 the base of the outer lip is the key to the 

 generic name, and the rounded, granite- 

 Fi s- 9- like appearance of the shell explains the 

 second, or specific name. 



It is a pretty little shell, about an inch in length, 



