552 THE BEGINNINGS OF LIFE. 



Thus the asexual modes of reproduction which exclu- 

 sively prevail amongst the multitudinous groups of 

 lower organisms included amongst the Protista, pass, 

 by the most easy gradations^ into the simplest kinds 

 of sexual generation ; and these simpler modes of sexual 

 reproduction gradually give place to more specialized 

 processes of the same kind. We find, moreover, that 

 individuals which have been produced by an asexual 

 process are often precisely similar to others that are 

 the products of fertilized germs. We find that the 

 asexual methods of reproduction predominate amongst 

 some creatures which are, nevertheless, occasionally 

 capable of multiplication by the higher sexual method. 

 And, lastly, we find even in the highest organisms which 

 habitually multiply themselves by the sexual process, an 

 asexual mode of multiplication occasionally taking place. 

 We have examples of this latter class of cases in the 

 ' Parthenogenesis ' which occurs in some of the Lepi- 

 doptera and other highest types of insect life j and also 

 in the fission of the early embryonic mass which seems 

 occasionally to take place during the origin of any of 

 the forms of the vertebrate series — not excepting Man 

 himself ^ 



The gradual interblending and difiFerentiation of the 

 asexual and of the sexual processes may be seen from the 

 following synoptical table, in which an attempt has been 

 made to classify the best known modes of multiplication 

 or reproduction which occur amongst living things: — 

 ^ Dr. Carpenter's ' Comparative Physiology,' 4th ed., pp. 480 and 580, 



