REPRODUCTION. 23 



two animals, one of which belongs to a lower morphological 

 type than the other, no degree of specialisation of function, 

 however great, will place the former above the latter, as far as 

 its type of structure is concerned, though it may make the 

 former a more highly organised animal. Every Vertebrate 

 animal, for example, belongs to a higher morphological type 

 than every Mollusc ; but the higher Molluscs, such as cuttle- 

 fishes, are much more highly organised, as far as their type is 

 concerned, than are the lowest Vertebrata. In a linear classi- 

 fication, therefore, the cuttle fishes should be placed above the 

 lowest fishes such as the lancelet in spite of the fact that 

 the type upon which the latter are constructed is by far the 

 highest of the two. 



It is obvious, therefore, that a linear classification is not 

 possible, since the higher members of each sub-kingdom are 

 more highly organised than the lower forms of the next sub- 

 kingdom in the series, at the same time that they are con- 

 structed upon a lower morphological type. 



TO. REPRODUCTION. 



Reproduction is the process whereby new individuals are 

 generated and the perpetuation of the species insured. The 

 methods in which this end may be attained exhibit a good 

 deal of diversity, but they may be all considered under two 

 heads. 



I. Sexual Reproduction. This consists essentially in the 

 production of two distinct elements, a germ-cell or ovum, and 

 a sperm-cell or spermatozoid, by the contact of which the 

 ovum now said to be "fecundated" is enabled to develop 

 itself into a new individual. As a rule, the germ-cell is pro- 

 duced by one individual (female) and the spermatic element 

 by another (male) ; in which case the sexes are said to be dis- 

 tinct, and the species is said to be " dioecious." In other 

 cases the same individual has the power of producing both the 

 essential elements of reproduction ; in which case the sexes 

 are said to be united, and the individual is said to be " her- 

 maphrodite," "androgynous," or "monoecious." In the case 

 of hermaphrodite animals, however, self-fecundation contrary 

 to what might have been expected rarely constitutes the re- 

 productive process ; and as a rule the reciprocal union of two 

 such individuals is necessary for the production of young. 

 Even amongst hermaphrodite plants, where self-fecundation 

 may, and certainly does, occur, provisions seem to exist by 

 which perpetual self-fertilisation is prevented, and the influence 



