Referate. 45 
existing bars have lost their effect, as is the case in genera where crossing 
between species is common. The latter is, as a rule, the case between the 
microspecies (Jordanons) of a species, the so-called “varieties”. 
In these rules we find also the explanation of the effect of domesti- 
cation upon the raising of new forms. The two factors, isolation and 
selection of the recessive types, are sufficient to explain the effect of 
domestication, but in many cases crossing with other forms and then 
segregation have added to the polymorphy of domesticated animals and 
plants. Using Darwin’s book on “Animals and Plants under Domestication” 
as reference, Lotsy shows that a mixed origin is the rule amongst the 
domesticated organisms. In many cultivated plants the “races” are really 
heterozygotes which can only be propagated vegetatively. But in all cases 
a cross is “at the bottom of is”; it is not the mere alteration of the outer 
circumstances which has called forth the “variability”. 
It was said above that crossing between species was an exception, but 
it does occur both in the animal and the vegetable kingdom. A. Kerner 
has first emphazised its value with regard to the evolution of plant species; 
but as Lotsy himself says in the preface, he has perhaps not laid enough 
stress upon Kerner’s importance as the first to advance hybridization as 
the main factor of the origin of new species. According to Lotsy hybri- 
dization is not merely the main factor, but the only known factor; and when 
it has been argued, that in this way we do not get new species with new 
characters, i. e. characters not present in the two parents, this is not correct. 
By crossing different species of Antirrhinum Lotsy has himself got types 
so much diverging from the parents that they ought really be taken as 
new genera. 
We can resume Lotsy’s reasoning in the following three sentences: 
hybridization is the cause of the origin of new types, heredity keeps them 
persistent, and selection is the cause to their dying-out. 
He does not flinch from explaining even the origin of the great 
systematical classes of organisms by means of his theory. From the palae- 
ontology we know that new classes appear suddenly with numerous, often 
highly differentiated types, but the more we remove in time from the origin 
of a class, the more the number of types diminishes and the remaining 
types become “reduced”. This is in good agreement with the theory that 
from a cross between two species, the offspring of the hybrid are at first 
very numerous and varied, but little by little the number gets smaller. until 
only a few types revive, or total extinction follows, 
“In the present time we live in a period, in which the extinction of 
many classes is almost completed and in which no new classes are formed”. 
(I do not think we are able to perceive if a new class is in statu nascendi). 
As an example Lotsy quotes Scott’s system of the vascular plants of which 
the majority of the divisions are either dying out (Equisetales, Psilotales, 
Lyeopodiales and Gymnospermae), or completely extinct (Pseudoborniales, Spheno- 
phyllales and Pteridospermeae), only two divisions (Filicales and Angiospermae) 
still flourishing. 
From his whole manner of argumentation it follows that he has not 
much good to say about relationship, phylogeny and homology. As to 
migration he does not deny that it has some value, but means it has been 
much overrated, as one has taken the monotopic origin of species for granted, 
while Lotsy means that the origin may be polytopic as well as polyphyletic. 
C. H. Ostenfeld. 
