THE CAUSATION OF DISEASE. 4 1 



duce the entire organism, but other parts of the plant possess 

 the same power ; thus the smallest part of a Begonian leaf 

 will grow into the entire plant, and, indeed, in some of the 

 lower plants, any single cell is alone capable of reproducing the 

 entire organism. 



Reproduction by Parthenogenesis is really asexual, although 

 in one sense it comes under the head of sexual reproduction. 

 In this method the organism is reproduced by the development 

 of an unimpregnated ovum. A partial development of the 

 ovum takes place, independently of impregnation, even in the 

 highest animals, in man for instance ; but complete develop- 

 ment of the ovum, independently of impregnation, has been 

 shown to occur sometimes in certain organisms. 



Reproduction by " Graft Hybridism." — This method consists 

 in grafting one variety into the stock of another. The blending 

 of the two tissues does not always occur, but in some rare 

 cases it does happen. This method is, strictly speaking, 

 asexual, but it approaches in character to sexual reproduction, 

 since it results from the blending of the tissues of two distinct 

 organisms — a hybrid mongrel resulting. It differs from sexual 

 reproduction in that the blending is not of two specific portions 

 of tissue thrown off by the reproductive organs, but of piece? 

 of tissue chosen more or less indefinitely. 



Partial Reproduction is always asexual. Broadly speaking, 

 this power varies inversely, as the complexity of the organism. 

 Nevertheless, some vertebrates possess it in a high degree ; for 

 instance, the tail and limbs of the same salamander have been 

 cut off, and have regrown, eight successive times. The healing 

 of wounds belongs, of course, to this method of partial asexual 

 reproduction. 



Sexual Reproduction is clearly allied to the above asexual 

 method. In parthenogenesis, for instance, we have a differen- 

 tiated portion of tissue, which is strictly speaking an egg, largely 

 dependent upon impregnation for development, but capable 

 sometimes of developing independently of such impregnation. 

 The process is in this case analogous to reproduction by budding. 

 Moreover, as just now observed, the hybrid in graft hybridization 

 results from the union of the tissues of two distinct plants. 

 Finally, in proof that sexual and asexual reproduction are* 



