DERIVATION OF ORGANIC BEINGS. 



became Twnioe (intestinal tape-worms) in carnivorous 

 animals."* 



3. But, although it would appear that we may safely 

 enough admit the universal derivation of living beings more 

 or less directly from others of the same kind, which stand 

 to them in the relation of parents, it is by no means so 

 clear that we are entitled to assume any absolute unifor- 

 mity in the way in which this law is carried into operation. 

 In fact, even a superficial survey of nature must make us 

 aware of one notable point of difference ; for, while in all 

 the higher forms, we find two parents (or their representa- 

 tives) concerned in the act of reproduction, we meet with 

 many cases among those lower in the scale of organization 

 in which a single individual appears capable of procreation 

 by its own unaided powers. It will suffice at present to cite 

 in illustration the case of the Aphides among insects. 



The existence of these two different modes of origin by 

 single and double derivation is now so universally 

 admitted that special terms are in use for their designation 

 such as homogenesis or monogenesis for the former, and 

 heterogenesis or digenesis for the latter, f The term gemma- 

 tion (budding) is also used by many authors to denote pro- 

 pagation by single derivation, as distinguished from that 

 higher form of generation which involves the combination 

 of two original elements. In the former mode of origin a 

 portion of the body of the parent becomes the seat of a 



* Address to Brit. Assoc., 1858. P. 34. 



f This is the sense in which the terms monogenesis and digenesis are 

 proposed by Prof. A. Thomson (Cyclop. Anat. and Physical, Art. Ovum, 

 Suppl. p. 42), and the sense in which they will be employed in the fol- 

 lowing pages ; but it is necessary to observe that Prof. Van Beneden, in 

 his extensive works on the reproduction of the Entozoa, uses them with 

 a very different meaning ; by monogenesis he understands direct develop- 

 ment ; and by digenesis the interpolation of intermediate forms, in the 

 way of alternation i.e. not as here, genesis from two origins, but genesis 

 in two stages. 



B 3 



