June 15, 1900.] 



SCmNGE. 



927 



do so, as they contain latent all the prop- 

 erties of the species, and are sometimes 

 able to manifest these properties (partheno- 

 genetic ova) without conjugating. In some 

 of the lower organisms the fact that conju- 

 gation does not result in an increase of the 

 number of individuals, but only in the pro- 

 duction of a new individuality, is quite ap- 

 parent, for in them two of the ordinary in- 

 dividuals of the species fuse to form one 

 (many Protozoa) . 



So that sexual reproduction gives us a 

 new individuality which can spread to al- 

 most any extent by asexual reproduction. 

 This asexual reproduction gives us a group 

 of organisms which is quite different from 

 a group of organisms produced by sexual 

 reproduction. Whereas the latter groups 

 constitute what we call species, the former 

 group has, so far as I know, no special 

 name, unless it be variety ; but variety is 

 not a satisfactory name, for it has been used 

 in another sense by systematizers. 



Heredity, then, is really applicable only 

 to the appearance in a zygote of some of 

 the properties of the gametes. A zygote 

 has this property of one of the precedent 

 gametes, and that property of the other, in 

 virtue of the operation of what we call he- 

 redity ; it has a third property possessed by 

 neither of the precedent gametes in virtue 

 of the action of variation, the nature of 

 which we have already examined. It is 

 impossible to say which property of a 

 gamete will be inherited, and it is impos- 

 sible to predict what odd property will re- 

 sult from the combination of the properties 

 of the two gametes. Of one thing only are 

 we certain, that they are never the same in 

 zygotes formed by gametes produced in im- 

 mediate succession from the same parent. 



We may thus regard the activities of the 

 zygote as the resultant of the dashing to- 

 gether of the activities of the gametes. 



Conjugation, then, is a process of the ut- 

 most importance in Biology ; it provides 



the mechanism by which organisms are 

 able to vary, independently of the condi- 

 tions in which they live. It lies, therefore, 

 at the very root of the evolution problem ; 

 the power of combining to form a zygote is 

 one of the fundamental properties of living 

 matter. 



SPECIES. 



N"ow let us consider one of the effects of 

 this property upon organisms. The effect 

 to which I refer is the division of animals 

 into groups called species. Species are 

 groups of organisms, the gametes of which 

 are able to conjugate and produce normal 

 zygotes. Now in Nature there appear to 

 be many causes which prevent gametes 

 from conjugating. First and most impor- 

 tant of all is some physical incompatibility 

 of the living matter which prevents that 

 harmonious blending of the two gametes 

 which is essential for the formation of a 

 normal zygote. Very little is known as to 

 the real nature of this incompatibility ; in 

 fact it is hardly an exaggeration to say that 

 nothing is known. It may be that there is 

 actual repulsion between the gametes, or it 

 may be in some cases, at least, that the 

 gametes are able to fuse, but not to undergo 

 that intimate blending which is necessary 

 for the production of a perfect zygote. In 

 some cases we know that something like 

 this happens ; for instance, a blend can be 

 obtained between the horse and the ass, 

 but it is not a perfect blend, the product 

 or zygote being imperfect in one most im- 

 portant particular — namely, reproductive 

 power. 



A second cause which prevents conjuga- 

 tion is a purely mechanical one — viz, some 

 obstacle which prevents the two gametes 

 from coming together. As an instance of 

 this I may refer to those cases amongst 

 plants in which conjugation is impossible 

 because the pollen tube is not long enough 

 to reach the ovule. In yet other cases con- 

 jugation is impossible because the organ- 



