Page:Philosophical Review Volume 14.djvu/371

355 by the first. Fluctuations are inadequate even to make a single step along the great lines of evolution. They occur only around a mean, obeying Quetelet's law of probability, and have never been observed to produce anything quite new.

But this is not the only objection to a theory of descent based on them. There is also the very serious one of incompatibility between such a theory of slow and nearly invisible yet accumulating changes and the results as to the age of the earth as given by the geologists and the astronomers.

The second division of lectures (II-IV) deals with 'Elementary Species in Nature.' Here and elsewhere it seems that de Vries distinguishes two ways in which species may originate: (a) by mutation, (b) by hybridization, according to the Mendelian law. The second presupposes, however, the constant unit-characters arising by the first method alone, and constituting elementary species. The test for a species of any kind consists in establishing the constancy of its unit-characters in pedigree-cultures.

Elementary species are not produced by man; nature alone does this by mutation. Their characters are not destroyed by intercrossing, but remain pure, and may be isolated whenever it is desired. But either as crossed or uncrossed, natural selection, by way of climate and soil and vegetable and animal enemies, operates as a sieve on them, so that some characters are kept, others lost.

In lectures V-XV, the author presents the evidence to show that 'varieties' are produced either by the loss of some marked peculiarity, or by latent characters becoming active, or by the acquisition of others that are already present in allied species. Only the elementary species form the progressive links of the chain from the lower to the higher forms; varieties are only local and lateral.

'No organism exhibits all of its qualities at any one time'; these may be dormant and awaiting a period of activity either regular or irregular. This means that unit-characters, having once been acquired, may become latent, and may reappear, and that this process is of universal occurrence throughout the whole vegetable and animal kingdom. The author accordingly finds that there are three aspects in the evolution at least of plants, viz., progression, retrogression, and 'degression.' In the first, there results an ever-increasing divergency; in the second, permanent latency, the diversity is increased; in the third, temporary latency, 'systematic atavism,' characters seemingly lost reappear. The test for the progression-constituting nature of mutations is a two-fold one. First, they distinguish themselves by falling outside the