REVIEWS. 
123 
believes the main divisions of the vegetable kingdom to have been 
built up. In the case of such mutations the author believes that the 
new character first comes into existence in a latent condition, and it 
may be only after many generations that it makes its appearance 
visibly—the period of mutation is preceded by a “ pre-mutation ” period. 
A second method of species formation, entitled by De Tries the 
degressive method, is indicated when a change takes place in the partial 
“ latency of a character. A completely latent character is indeed 
unrecognizable as such. But characters may be partially latent, and 
in this case they exhibit themselves from time to time in rare individuals 
in the form of abnormalities or sports. An active character, on the 
other hand, is apparent in the great majority of individuals of a race. 
If a change from latency to activity occurs suddenly, this is a form 
of mutation. On the other hand, a character previously active may 
become latent ; the character then appears to be lost and the mutation 
is said to be retrogressive. De Vries regards the great variety of allied 
species which occurs in many groups as being largely due to retro¬ 
gressive mutation. It is also frequent in garden culture, where the 
appearance of a white variety of plant, previously only known to 
produce coloured flowers, may be a good example of a retrogressive 
change. Mutations may also be atavistic , the usual case being that 
an ancestral character which had become latent shows itself once 
more in the active condition. Finally, new forms may arise by the 
intercrossing of distinct species. 
The greater part of the volume is occupied by an account of the 
experimental evidence upon which the conclusions summarized above 
are based. It is impossible to give here more than the barest idea of 
these observations. The main conclusions are based upon experiments 
carried out with plants of CEnothera Lamarckiana, which occurred in 
great numbers as “ escapes ” near Hilfversum, and which, according to 
De Vries’s interpretation of the facts, were discovered in a period of 
mutation. 
In the locality mentioned two forms of (Enothera, previously 
unrecorded, were found growing in company with their supposed 
parent ; each of them on being tested came true to seed. Seed and 
roots of 0. Lamarckiana were then taken and grown in the garden, and 
here a number of other new forms arose. In eight generations, starting 
from O. Lamarckiana in 1886, about 50,000 individuals were grown to 
a recognizable stage, and of these over 800 showed mutation, i.e., 
1*5 per cent., and this estimate is likely to be too low rather than too 
high. 
The new forms numbered about 15, and most of them appeared 
repeatedly, but some were much more frequent than others. Many 
were recognizable as quite young seedlings, notably O. albida ; others 
not until a considerably later period of growth. Having once appeared, 
the majority of new forms came true to seed. Sometimes among their 
