258 THE EVOLUTION THEORY 



in the germ-plasm. But there seems to me to be a difference and 

 one which is not without importance. 



In parthenogenesis no amphimixis occurs, but neither does any 

 reduction of the number of the ids to one-half : all the ids present at 

 the beginning of parthenogenesis are retained ; they are onl}' no 

 longer mingled with strange ids. In inbreeding both amphimixis 

 and reduction take place, but the former soon ceases to convey an}' 

 really strange ids to the germ-plasm, but only the same as those 

 wdiich it already contains, so that a rapidly increasing monoton}' 

 of the germ-plasm must result. To this must be added the possibility' 

 that among the few ids which now — many times repeated — form the 

 germ-plasm, some must occur which exhibit unfavourable variational 

 tendencies in one or many determinants, and then the same thing 

 will occur which usually occurs in experimental inbreeding of 

 domesticated animals, namely, degeneration of the 'p^^ogeay. In 

 parthenogenesis the case is otherwise ; unfavourable variational 

 tendencies, as soon as they attain selection- value, are, so to speak, 

 eliminated root and branch, because the individuals which exhibit 

 them, and their whole lineage, are exterminated, without their having 

 any effect upon the other collateral lines of descent. A purely 

 parthenogenetic species will, therefore, not degenerate as long as 

 individuals of normal constitution are present, for these reproduce 

 with perfect purity. But if in later generations unfavourable 

 variational tendencies crop up in the germ-plasm through germinal 

 selection, the process of personal selection will be reinforced on these 

 or on their descendants, and it is conceivable, and even probable, that 

 in perfectly adapted species parthenogenesis may last for a ver}' long 

 time without doing any injury to the constitution of the species. 



The same is true of purely asexual reproduction, to the 

 investigation of which Ave shall now turn. 



Let us leave out of account the simplest animals (Monera) without 

 amphimixis, which we have already discussed. In simple animals 

 reproduction by budding or by fission is frequent, or it occurs in 

 alternation \A'ith sexual reproduction ; in higher animals. Arthropods, 

 Mollusca, and Vertebrates, asexual reproduction is wholly absent. 

 In plants it plan's an enormousl}^ greater part, and what is called 

 * vegetative reproduction,' which is purel}^ asexual Avithout an}' 

 amphimixis, is to be found in all groups of plants, especially- in the 

 form of budding and spore-formation, besides which there is mul- 

 tiplication by runners, rhizomes, tubers, bulbs, and bulbils. In most 

 cases there is, in addition to the purely asexual reproduction, so-called 

 sexual reproduction associated with amphimixis, and often the sexual 



