CHAPTER IX 

 POLYMORPHISM AND LIFE-CYCLES OF THE PROTOZOA 



A. POLYMORPHISM. 



ONE of the most striking peculiarities of living beings is the infinite 

 variety of form, structure, and appearance, which they present. 

 There is, perhaps, no living individual of any kind which is exactly 

 similar, in all respects, to any other. Nevertheless, the most 

 uncultured intellect cannot fail to recognize that, in the case of 

 all ordinary, familiar plants and animals there is a pronounced 

 tendency to segregation into distinct kinds or species that is to 

 : ay, natural groups of individuals which, though they may vary 

 greatly amongst themselves, yet resemble one another far more 

 than they do the individuals of another species. It is not necessary 

 to point out that species are not to be regarded as permanent or 

 immutable entities. It is certain that a species maj T in course 

 of time become modified so as to acquire characters different from 

 those it originally possessed, thus giving rise to a new species, or 

 that a single parent-species may become split up into a number of 

 groups which, by a similar process of modification, became so many 

 daughter-species differing from one another and from the parent- 

 species to a greater or less degree. The problem of the origin of 

 species is one that it is not necessary to discuss here ; it is sufficient 

 to point out that the mutability of species often makes it very 

 difficult to define or delimit a given species exactly, of which a 

 striking example is seen in the pathogenic trypanosomes of the 

 brucii- group, probably to be regarded, as pointed out above 

 (p. 27), as instances of species in an incipient or nascent condition. 

 Some species are sharply marked off from others, some are much 

 less so, and some are of questionable rank, regarded by one naturalist 

 as distinct, by another as mere races or varieties- a state of things 

 perfectly intelligible if existing species are regarded as having 

 arisen by descent, with modification, from pre-existing species. 



In the Protozoa the existence of distinct species is just as marked 

 as in the higher plants and animals, and is universally recognized. 

 As has been pointed out in the previous chapter, it is probably 

 syngamy which is responsible for the segregation of individual 



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