l.EOTUEE XXX 



INBREEDINC!, PAUTIIENCH^ENESIS, ASKXUAL REPR()DU<"- 

 TION, AND THEIU (n)NSl<:giTENCKS 



Tlui aopnriil ion of tlio soxom dxisl.H (iv<iii among tlio Protozoa — Gonilitions di'ldnnin- 

 iiiK the ooouvn'iifo of llnniiaiilu'oditism — 'I'liiiivwiiniiM, Oin'ln|)oilH — rrimordial nialcH 

 — Advantasi'H of pai'tlinuogonosis- AUrcrn.'iliuii willi bi-scxual geiionil.ioiis — In Oall- 

 wasjis — In Aphides — OrdSK-forlilizalinn Mccurod in plants- — Solf-fertilizalion is avoidod 

 whenovoi' po9silili>— Tlic moohanism nf fortili/.alion and tlio mingling of gorm-plasniM 

 must be oloarly distinKMixliod from oni' iinntlun-— (liisog of iiorsistdiil Holf-fprtilization — 

 Tlio effects of inlirooding cimiparod with those of piul.lionogonosis— Tim oflfoct of purely 

 asexual rciiroduction — In sea-wraolts — In liclioiiM and fungi — In cnltivated plants — 

 I )( 'generation of tlio sox-orgaiin — Summary, 



We have seen tliufc continued inbre(>iliiig niUHt make the o;enn- 

 plasiii monotonous, and tluM-el'oni iniplastic as rcf^firds tlie reqnirctiuMitH 

 of adaptation. Accordingly, wc^ found that tlu^ ganietcH of many 

 unicellulars art^ so constituted tliat tliey oidy possess a power of 

 attraction for gametes of a dittei'ent lineage, not for tlioso of tiiiMr 

 own stock. Among multicellular organisms the most intense mode 

 of inbi-ecding is to bo found in the uninterrupted self-i'ertilisiation of 

 hermaplirodites : in such eases (.lie monotony of the genu-pliisiii 

 must ri^ac'li extreme ex]ireHsion more rea,di]y than in the case of 

 ordinary inbi'eeding. We can thus understand why, in the scale 

 of organisms, there is such an early occurrence of gonocliorism, the 

 separation of tiie species into male a.nd fi^male individuals. Even 

 among unicellular plants or I'l-otophytes this occurs oecMsionally, as 

 it does ill the Vorticellids among Infusorians. 



In the JVit^tiizoa and Metaphyta, the separation of the sexes finds 

 emphatic oxpri^ssion ; it is absent from no important group, and 

 in many, such as, for instance, among the Vc^rtebrates, it lia,s become 

 the absolutely noi-mal condition, with hardly any exc,e|iti(Hi. But 

 in many divisions of the animal and plant kingdoms hermaphroditism 

 also plays an important jiart, as, lor instance, in terrestrial snails a.nd 

 in flowering plants. 



Obviously the sexual adaptations of a. species a,re di^iinitoly 

 reJatinl to the conditions of its life, and, though Nature's endi^avour 

 to prevent inbreeding and to secure cross- fertilisation is evidi^nced 

 by the occurrence of si^parato sexes in such a multitude of forms 



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