7° THE .ORCHID REVIEW. [MarCH, 1914. 
Casts eat | 
KONG KOK) 
: ORIGIN OF SPECIES BY CROSSING. ” 
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HE meeting of the Linnean Society held on February roth, at which a 
paper was read by Dr. John P. Lotsy, of Haarlem, ‘‘ On the Origin 
of Species by Crossing,” illustrated by diagrams, lantern-slides, and dried 
specimens, was the longest and most crowded in the Society’s history, the 
subject occupying the whole evening until after eleven o'clock. 
The lecturer said :-—We have in all questions of evolution to gather our 
facts from individuals, because species as well as varieties are abstractions, 
not realities. Nobody is able to show you a species ora variety ; all he can 
do is to show you one or more individuals which he believes to belong to: 
the species or variety under discussion. 
Of individuals we know two kinds; homozygotes and heterozygotes,. 
The first are stable, the latter segregate, earlier or later, into new. 
homozygotes. The offspring of a homozygote is identical with its parent: 
with the exception of mere temporary, non-transmittable modifications. If 
this be.true, selection in.the progeny of a definite homozygote can have no: 
effect. That it has no-effect has been proved by Johannsen. ' 
A homozygote consequently is absolutely stable and produces offspring 
which are genetically identical to it. Yet not all homozygotes are the 
same, there are many different kinds of homozygotes: homozygote beans, 
homozygote Antirrhinums (of which ] shown), homozygote dogs, 
amples 
&c. All these different kinds of homozygotes, we may call, with Johannsen,. 
genotypes, because they differ in genetical constitution, and we can them: 
say that the world is populated—with the exception of heterozygotes—by a 
large number of sharply-defined absolutely stable genotypes. Under such: 
conditions evolution may well seem impossible ; fortunately, the behaviour 
of the heterozygotes shows us that it is very well possible. 
A careful study of the descendants of a heterozygote shows us that it: 
segregates in the next. or later generations into a number of individuals, 
part of which are heterozygous, but part of which are homozygous, and 
that these homozygotes belong to different genotypes. A heterozygote 
consequently gives birth to a smaller or larger number of different 
genotypes. By carefully watching therefore a heterozygous individual we 
see the origin of genotypes. 
The next step is thus to produce at will these genotypes—originating 
heterozygotes. This we can do by crossing two individuals belonging to- 
different genotypes. 
The next question is: do all heterozygotes obtained by crossing segregate 
and thus give rise to different ‘genotypes? Until very recently it was- 
believed that only heterozygotes obtained by crossing so-called varieties did. 
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