472 



LIFE : OUTLINES OF GENERAL BIOLOGY 



gamut, wo must bo wry careful in arguing from one level to 

 another. What is normal and apparently wholesome at one grade 

 may be very undesirable at another. 



It has been securely established that some hermaphrodite animals 

 habitually fertilise their own eggs. This "autogamy" has been 

 proved in some tapc^worms and flukes — not auspicious illustrations; 

 it seems sometimes to occur in the freshwater Hydra and a few other 

 free-living animals. There are numerous self-fertilising flowers, 

 though there is no case known where cross-fertilisation is impossible. 

 It may also be that one hermaphrodite liver-fluke sometimes insemi- 

 nates another, so that the habitual autogamy may be interrupted. 

 In the great majority of hermaphrodite animals, such as earthworms 



Fig. 63. 



Male and l-Vmalc Or^an-s of a Simple Alga ((Kdogonium). aft', antheridium 

 liberating spermatozoa; oog, female organ or oogonium containing an 

 ovum; an, an antheridium empty. 



and snails, cross-fertilisation is the invariable rule. It is also relevant 

 to recall the fact that in many of the small Crustaceans, in many 

 Rotifers, and in some insects, such as Aphides, there may be long- 

 continued parthenogenesis- generation succeeding generation with- 

 out loss of vitality, although tho eggs develop without any fertilisa- 

 tion. In some of tlu' Rotifers the males are still undiscovered; 

 Reaumur kept .\phides breeding ]xirth( nogetically for over three 

 years {50 consecutive generations), and W'cismann kept females of 

 a common water- flea {Cypris rcptans) breeding in the same way for 

 eight years. This shows that at certain levels of organisation a vigor- 

 ous life may hv. kept up for many generations, not only without 

 any introduction of "fresh blood", but without the presence of any 

 males. 



A numl>cr of careful e.\|)eriments have been made on inbreeding, 

 but there is imjxTative need for more. Wiismann in-bred mice for 

 twenty-nine generations, and his assistant. Von Guaita, continued 



