4 88 POPULAR SCIENCE MONTHLY. 



Yet differences between species are often so small that only a very 

 careful study can make us acquainted with them. Among our native 

 plants I have but to mention Cochlearia Anglica and C. danica, Lepi- 

 gonum [Spergularia~\ salinum and L. medium, Chrysanthemum mariti- 

 mum and C. inodorum, Car ex Oederi and C. flava. These are differ- 

 ences which one would rather neglect. Other examples can be met 

 with in the genera Rosa, Rubus, Salix, Hieracium and many others; 

 each botanist is acquainted with them, they are the common stumbling- 

 blocks on botanical excursions. Yet in systematic botany they are 

 regularly recognized as bona fide species. 



It sometimes occurs that two of these species which closely re- 

 semble each other grow side by side, as in the instances above-men- 

 tioned. In this case one can as a rule compare them when fresh, and 

 in this manner fully realize the differences existing between them. 

 But it happens far more frequently that the two plants, or three or 

 four members of a small group, occur in different countries, often 

 at great distances from one another. Then the differences are far 

 less apparent. To this must be added that by the drying process neces- 

 sary for herbarium purposes, many characters are lost. In that case 

 the plants are no longer clearly distinguished, and are ordinarily con- 

 sidered as a single species, united under one name. This happens with 

 Draba verna, Viola tricolor, Helianthemum vulgare and numerous other 

 plants. It is only when we obtain them from different countries and 

 grow them next to each other in the garden that the differences become 

 apparent, and it is only then that these differences prove to be as great 

 as those existing between the members of the above-mentioned couples 

 of species. 



One must therefore consider each mutation a step not greater than 

 the differences between Chrysanthemum inodorum and C. maritimum 

 for instance. I choose this example because the first species, the 

 double form, with entirely filled, pure white, exceedingly graceful 

 heads, is a well-known component of bridal bouquets. Besides, both 

 are native species and of common occurrence, but generally not dis- 

 tinguished on botanical excursions. Where the differences between 

 related species are greater, the lack of transition forms must be at- 

 tributed to the fact that these live in other countries, or to their having 

 become extinct. 



In the second place, various investigators have come to the con- 

 clusion that mutations must occur periodically. For it is only in this 

 manner that we can make the theory of descent agree with the unde- 

 niable fact that the species, such as we know them at present, have 

 remained unchanged for centuries. In certain localities, on islands 

 for instance, or places so situated that for centuries no transportation 

 of plants or seeds can have taken place, the individuals of any one 



