238 THE NATURE AND ORIGIN OF LIFE 



be easily discovered under the microscope, and yet the 

 sudden disappearance or introduction of one of these 

 microbes in an individual might determine a considerable 

 and hereditary morphological variation. In this hypothesis 

 a mutation would not be the sudden appearance of a species, 

 but the formation or destruction of a new association 

 of two or more species already known. 



In the present state of the question, we have no reason 

 to adopt one explanation rather than another. The ex- 

 planation by parasites is related with a very frequent 

 phenomenon in the vegetable world the formation of 

 galls, of which the best-known is that of the oak or nut- 

 gall. 



A gall is a local deformity of the plant, produced by the 

 local action of an animal or vegetable parasite (in the case 

 of the oak the nut-gall is produced by the development of 

 eggs laid by an insect). This local deformity has great 

 importance in biology, as may be seen from what has been 

 said concerning morphogemc or formative diastases in 

 connexion with the morpho-biological theorem. Here 

 we see how one or more diastases emanating from the para- 

 site are added, in the surrounding vegetable tissues, to the 

 morphogemc diastase of such tissues and determine a new 

 morphology in the infected region. The gall thus pro- 

 duced has a double specific quality, that is, it derives its 

 characters at once from the species of the host and the 

 species of the parasite. Two different insects produce in 

 the same plant two different galls. One and the same 

 insect, when it is capable of developing in two different 

 plants, also produces in these different plants different 

 excrescences. There is here a whole chapter of biology, the 

 descriptive part of which has even been given a special 

 name Cecidiology, or the description of galls. 



