10 DERIVATION OF ORGANIC BEINGS. 



certain independent manifestation of vitality, and a focus of 

 such intensity of the plastic processes, that, in the course 

 of time, the part is converted into a distinct organism, 

 capable of detachment from the parent, and fitted to main- 

 tain a separate existence. Such a detached gemma may be 

 termed 2^ free zooid ox phytoid. 



In the ordinary form of reproduction again — that by the 

 co-operation of the sexes — a fusion seems to take place of 

 two highly vitalized portions of the same or kindred organ- 

 isms, which results in the formation of a fecundated germ, 

 possessed henceforth of an independent vitality, endowed 

 mth a capacity for ultimately acquiring the structure 

 characteristic of the species, and destined to be thrown on 

 its own resources, by its extrusion from the protecting 

 envelopes, as soon as its organization is sufficiently ad- 

 vanced for this condition. In all but the very lowest forms 

 of life — the conjugating Algse — a difference is observable 

 between the two factors of the embryonic product, which 

 are recognized respectively as male and female, or as the 

 spermatic and germinal elements. 



§ 4. It has long been known that these two modes of 

 propagation may co-exist in the same species — ^the plant or 

 animal multiplying now in one way, now in the other ; but 

 the general relations of the two modes of increase to each 

 other, throughout series of organised beings as a whole, has 

 only recently engaged the attention of naturalists. Till 

 lately, the general opinion seemed to be that the latter 

 form of reproduction was the normal one in the higher 

 species both of plants and animals, while derivation from a 

 single parent prevailed in the lower grades of both king- 

 doms, there being some in an intermediate position, which 

 furnished examples of the co-existence of the two forms. 

 This arrangement was considered to be quite exceptional 

 among animals, though of common occurrence, under some 

 modifications, in the vegetable kingdom ; but no farther 



