748 THE AMERICAN NATURALIST. [VoL. XXXIX. 
It is a matter of congratulation that we should now have, in English, 
a discussion of the mutation theory and the line of thought which 
led the author to put it forward, as well as an account of the experi- 
ments on Lamarck’s Evening Primrose which has figured so largely 
in the habilitation of the theory. Not indeed that this volume is to 
be regarded as a translation of his Muzations-theorie, for without the 
great amount of detail found in that work it treats the subject in a 
broader way, aiming to bring out the mutation concept with sufficient 
specific illustration to support the principles enunciated. After the 
introductory chapter, which is of a historical nature, the discussion 
of what are termed elementary species affords the topic of several of 
the succeeding ones. Here we find a highly illuminating treatment, 
which, in connection with what is said later about varieties, serves to 
give a clarified conception of the import of the Linnean group or 
collective species. That the latter are, indeed, an abstract idea, an 
average as it were of a number of types grouped together, is brought 
sharply to the attention. The modern taxonomist who is in sym- 
pathy with this idea must at once see a new significance to the 
segregations whicb are constantly and often of necessity being made 
from the older forms which have been considered species. Elemen- 
tary species are contrasted with varieties, in that they differ in more 
than one respect and possess qualities which are distinctly new, 
while in varieties the whole character difference may usually be 
expressed by a single term and which, as the evidence goes to show, 
do not possess any really new qualities. Most varieties arise in a 
negative way, by the loss of some quality, and much more rarely do 
they appear to be positive, possessing some feature new to them- 
selves, which, however, is to be regarded as the reappearance of a 
character seen in allied forms. Moreover, these characters are, 
whether of à positive or of a negative nature, physiologic units and 
appear and disappear singly. From this the development of the 
author's line of thought leads him to the consideration of atavism 
and the associated subject of hybridism. In discussing true atavism 
all cases which might be confused with it that arise from hybridism, 
or from varieties which annually produce sports, are to be excluded. 
Atavistic tendencies in this strict sense are found to be very rare, 
and while reversionary forms may often be widespread in the sense 
of ERBEN aphical distribution, it is found that this is due to bud propa- 
gatıon. For what the author calls false atavism he introduces a new 
dps Ee uae ee to carry the idea of chance spatial contiguity, 
which results in spontaneous and often undetected crosses. Under 
this head then will come many of the instances of so called atavism. 
