IT. Organs. 10. Reproduction and Reproductive Organs. 37 



Under natural conditions, hybrids occur relatively seldom. On the border- 

 land between the ranges of two species (see section Y.), hybi-ids are, indeed, 

 ■often found, and in some groups (Fresh-water Trout), seem fairly common. The 

 interbreeding of species is hindered, amongst other things, by the fact that the 

 individuals of different species, at least in a state of nature, are generally 

 disinclined to pair. 



Just as the offspring of a union between remote species is unhealthy, so 

 also is that produced by too close in-breeding; if the offspring of the same 

 parents, or if other near relations are mated, and this close breeding continues 

 for several generations, the descendants gradually show a marked deterioration 

 ■of one kind or another. 



Parthenogenesis. — The development of an ovum into a new 

 individual is generally dependent^ as already stated, on its being 

 fertilised : but in not a few animals, it has been observed that the 

 ovum can develop without fertilisation. This peculiar 

 modification of sexual reproduction, which occurs in Platyhelminths, 

 Insects, and Crustacea, is called parthenogenesis, and although 

 in some forms where the unfertilised ova usually atrophy, it may be 

 •exceptional (Silk-worms), yet in other cases it is quite an ordinary 

 phenomenon. In many species there are whole generations which 

 ■consist exclusively of females, so that the eggs cannot possibly be 

 fertilised, and yet they develop nevertheless (Aphides). Amongst 

 ■other forms the male seldom or never appears. In Bees and other 

 Hymenoptera, all the fertilised eggs develop in a remarkable way 

 into females, whilst the unfertilised ova become males [see also the 

 Special Part, Trematoda, Aphides, Daphnidse, and Inseota) . 



In some animals, although parthenogenesis does not occur regularly, it is yet 

 .suggested. It has been found that development begins in an unfertilized Hen's 

 egg for instance, but does not go beyond the very first stages (segmentation), 

 which are followed by degeneration. The same thing has been observed in the 

 ■ovum of the Eabbit. 



Ova, which are, as it were, predestined not to be fertilised ; which, for example, 

 are produced from generations consisting only of females, are peculiar in giving 

 ■origin to o n e polar body only (summer eggs of Daphnia). It is a question 

 whether the eggs of Bees, which sometimes are fertilised and sometimes are not, 

 throw off two polar bodies as is customary. It has been observed in some ova, 

 which are usually fertilised, but which can develop parthenogenetioally to a 

 ■certain point, that the nucleus for the second polar body always forms ; but if 

 fertilisation does not take place, it unites again with the egg nucleus, and 

 subsequently plays the part of the missing male pronucleus (Round-worms, 

 Star-fish). 



Alternation of Generations. — Some animals reproduce both 

 ■sexually and asexually : many Corals can produce new individuals by 

 budding as well as by ova and spermatozoa, and this is true also of 

 ■certain Annelids and Ascidians. In other cases, however, those 

 individuals which give rise to buds, do not produce sexual cells; 

 asexual reproduction is restricted to some members of the species, 

 sexual to others, and there is a more or less regular alternation 

 of sexual and asexual broods or generations ; asexual individuals 

 produce, by gemmation or fission, sexual individuals whose fertilised 



