REPRODUCTION. -> r 



/ 



two animals, one of which belongs to a lower morphological f 

 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. 



In the words of Professor Allen Thomson, " it has become 

 more and more apparent in the progress of morphological 

 research, that the different groups form circles which touch 

 one another at certain points of greatest resemblance, rather 

 than one continuous line, or even a number of lines which 

 partially pass each other." In the same way the highest 

 vegetables do not approximate to, or graduate into, the lowest 

 animals ; but, " each kingdom presents, as it were, a radiating 

 expansion into groups for itself, so that the relations of the ' 

 two kingdoms might be represented by the divergence of lines 

 spreading in two different directions from a common point." 



10. 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. Seocual Reprodtiction. 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 



