species do not show any or, at least, no constant differences, the above-mentioned couples of species, and compound species excepted. Spruces form a compound species, consisting of numerous types, but the common fir which without doubt is older than our era has remained the same everywhere. It is ever thus; the species do not undergo any gradual change, but each species is constant and remains so until others take its place. It never or but rarely occurs that new species make their appearance in fully investigated countries, unless indeed they happen to have been introduced from elsewhere. Yet it is probable that new species are formed quite frequently, but that, being too weak, they succumb before one becomes aware of their existence.
The numerous small species which are united under the name Draba verna are constant to seed, they do not change, besides they are distributed throughout Europe. It is therefore considered probable that there was a time during which they were formed, probably in a comparatively small region in the central part of Europe [at the present day they are most frequent along the Rhine and the Loire], and that in this locality flourished one or more species from which the present forms originated. At the end of this mutation-period the species would again have become constant. In this manner mutable and immutable periods in the development of species would have alternated more or less regularly.
There is a great tendency to consider a rapid increase in number as one of the reasons which cause a species to become temporarily mutable. Many species multiply exceedingly rapidly when they are transported to a new region where the conditions are favorable. Many European plants did this in America, likewise many American plants in Europe, as is only too well known through the waterpest, Elodea canadensis. As a matter of fact, we did not see them 'mutate,' but this may have been due to insufficient observation. It would be of great importance to pay close attention to this point when draining lakes, clearing waste lands, after forest fires and in similar cases.
Whether the mutations, during the mutable period, have been one-sided or many-sided, is a most important question and one frequently discussed by the adherents of the mutation theory. The case of Draba verna, just mentioned, certainly speaks in favor of many-sided variability; the 200 'subspecies' known, vary in all organs and in all possible ways. Numerous other instances might be quoted. But opposed to them are the results obtained by paleontology. The progress in zoological times, more particularly in the animal kingdom, has always followed definite lines; by a straight line nature tried to reach her goal, not by zigzag lines, feeling her way. The main line has of course numerous small side branches, but branches which do not lead to still living types are rare. Scott and others deduce from this that mutability is one-sided, only progressing in the desired direction. Yet
