POLYMORPHISM AND LIFE-CYCLES 163 



into species, by blending the divergent characters that may be 

 supposed to arise from the influence of different conditions or 

 circumstances of life. Thus, syngamy in unicellular organisms 

 appears to have an effect which is the opposite, to a large extent^ 

 to that which it produces in multicellular organisms, in which there 

 are special germ-cells, sheltered to a greater or less degree from 

 the direct influence of the environment, and in which amphimixis 

 appears rather to be a means by which variations arise. 



The conception of a species is by no means incompatible with the 

 occurrence of a number of distinct forms in its life-history. Taking 

 well-known instances from the Metazoa, there may be, in the first 

 place, ontogenetic or developmental differences ; not only may the 

 individuals of the same species differ in size at different periods in 

 the development, but they may differ so greatly in appearance and 

 structure that only a knowledge of the life-history enables us to 

 assert that they belong to the same species as, for example, a 

 caterpillar and a butterfly, or a hydroid and a medusa. Secondly, 

 the adult individuals may differ to an enormous extent in the two 

 sexes. Thirdly, there may be in many cases differences between 

 individuals of a species related to differences in the functions which 

 they perform, not merely at successive phases in the life-history, 

 as in some cases of ontogenetic differentiation already mentioned, 

 but even at corresponding phases of the life-history a phenomenon 

 best seen in social or colony-forming organisms, as in the case of 

 ants and termites, or in the colonies of Hydrozoa. 



In Protozoa, similarly, a given species may show distinct phases 

 or forms at different or corresponding periods of its life-history to 

 a greater or less extent. In some species the form-changes are very 

 slight, and the individuals occur always under a similar form and 

 aspect, at least during the active state, and are therefore recog- 

 nizable without difficulty as regards their specific identity ; such 

 forms may be termed monomorphic, and as examples the species 

 of ciliate Infusoria can be cited. Other Protozoa, on the other hand, 

 are extremely polymorphic that is to say, they occur under a 

 variety of widely-differing forms at different stages in the life-cycle 

 or in response to variations in the conditions of life. Hence it is 

 often difficult or impossible to refer a given form to its proper 

 species without tracing out its life-history and following its develop- 

 ment step by step. The unravelling of the complicated life-cycles 

 of Protozoa is attended by far greater difficulties than in Metazoa, 

 since one important criterion fails us altogether in the Protozoa, 

 that, namely, of sexual maturity. A naturalist has no hesitation 

 in pronouncing a trochophore to be a larval form, and a rotifer to 

 be an adult organism, from the fact that the former is sexually 

 immature, while the latter produces ripe generative cells. In the 



