Page:Popular Science Monthly Volume 62.djvu/494

488 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, Lepigonum [Spergularia] salinum and L. medium, Chrysanthemum maritimum and C. inodorum, Carex Oederi and C. flava. These are differences 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 stumblingblocks 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 resemble each other grow side by side, as in the instances above-mentioned. 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 necessary for herbarium purposes, many characters are lost. In that case the plants are no longer clearly distinguished, and are ordinarily considered 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 distinguished on botanical excursions. Where the differences between related species are greater, the lack of transition forms must be attributed 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 conclusion that mutations must occur periodically. For it is only in this manner that we can make the theory of descent agree with the undeniable 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