THE ORIGIN OP SPECIES 175 



LiCHENES. 



Peltigera canina Hoffm. Gladonia fivibriata var. conista 

 P. rufescens Hoffm. Nyl, 



P. phy socles Ach. C. gracilis Hoffm. 



Cetraria aculeata Fr. C.furcata Hoffm. 



Gladonia pyxidata Fr. C. furcata var. corymhosa Nyl. 



C. pyxidata var. pocillum Fr. C. pungens Nyl. 



C. chlorophcBa Flk. Cladina sylvatica Nyl. 



C.fimhriata Fr. f. exilis (Ach.). C. uncialis Nyl. 



C fimbriata var. tuhcBformis Bilivibia ligniaria Massal. 



Fr. Lecidea idigiiiosa Ach. 



THE OEIGIN OF SPECIES. 



[The following is an abstract of a paper on " The Origin of 

 Species by Crossing " read at a meeting of the Linnean Society 

 on the 19th of February, by Dr. J. P. Lotsy, of Haarlem. The 

 paper was illustrated by diagrams, lantern-slides, and dried 

 specimens.] 



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 or a 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 hetero- 

 zygotes. The first are stable; the latter segregate, earlier or later, 

 into new homozygotes. The offspring of a homozygote is identi- 

 cal with its parent, with the exception of mere temporary non- 

 transmittable modifications. If this be true, selection in the pro- 

 geny of a definite homozygote can have no effect. That it has 

 no effect has been proved by Johannsen. A homozygote con- 

 sequently is absolutely stable and produces offspring which is 

 genetically identical with it. Yet not all homozygotes are the 

 same, there are many different kinds of homozygotes : homo- 

 zygote beans, homozygote Antirrhinums, &c. 



All these different kinds of homozygotes we may call with 

 Johannsen genotypes, because they differ in genetical constitu- 

 tion, and we can then 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 quite possible. 



A careful study of the descendants of a heterozygote shows 

 us that it segregates in the next or later generations in 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. 



