204 



HABIT AND INTBLLIGENCE. 



[chap. XVI. 



Their 

 meta- 

 genesis. 



explanation of what formerly was tlie most perplexing 

 question in biological science — I mean the origin of the 

 internal parasites of animals. The germs of these, I have 

 no doubt, have been in all cases originally introduced from 

 without, either in the food or through the skin, and have 

 been developed into different forms from those which they 

 would have assumed had their development taken place 

 elsewhere. This conjecture is almost proved by the dis- 

 covery of the fact that many of these internal parasites, or 

 Entozoa, alternate by metagenesis ^ with animal forms that 

 inhabit the earth or the water. 



NOTE. 



INDIVIDUALITY. 



Indi- 

 viduality, 

 difficult 

 to define 

 among tbe 

 lower 

 orrjauisms. 



Morpho- 

 logical 

 units of 

 different 

 orders. 



Among the lower organisms, it is nearly impossible to define 

 what an individual is. The single cell of one of the lowest 

 Algse can scarcely be described as other than an individual, and 

 yet it is homologous with the cells that combine to form the 

 filaments of zygntma, or the fronds of the higher Algse. The 

 separate hydra is homologous with each polypite of the com- 

 pound Hydrozoa ; yet the latter are united into one compound 

 organism, in much the same way that the leaves of a tree are 

 united. In fact, the word " individual " is scarcely applicable ; 

 the expression, " morphological unit," proposed by Herbert 

 Spencer, is better, as we can speak of morjihological units of 

 different orders. Among the higher orders of plants, the mor- 

 phological units of the first order are the cells ; these, by com- 

 bination, form the units of the second order, which are the 

 leaves with their stalks ; these combine into units of the third 

 order, that is to say, into branches, or axes (understanding by 

 this term the product of a single bud), and the branches into 

 trees : just as the unit of organization in an army may be suc- 

 cessively taken to be the individual soldier, the company, and 

 the regiment. 



1 Metagenesis, a parallel word to metamorphosis, and signifying that 

 two forms produce each other alternately. Thus, to take an instance from 

 a different class, many hydra-like forms produce medusae, and the medusee 

 again produce hydra-like forms. 



