PEOPLING OF LAND AND SEA 159 



security therein in a consequence at first sight most singular. 

 Many became hermaphrodite, for instance, the Oligochaetes, 

 the Leeches, ancestors of the flat worms (Trematodes, Cestoides, 

 Turbellarians) , and Pulmonate Gasteropods. Whatever may 

 be the general belief, hermaphroditism is not a primary condition . 

 The primitive genital elements, the spores, were simple asexual 

 cells and in the first place they multiplied directly without 

 fecundation. This is still the case with many cellular 

 Cryptogams and certain Protozoa, but even in these groups 

 sexual differentiation of the cells has already appeared as well 

 as fertilization in the ciliated Infusoria, for example, in the 

 Sporozoa and Foraminifera. When certain genital cells develop 

 without fertilization, as in Apus, Branchipus, Daphnia, the 

 Aphides, Cochineal-insect, the Cynipidae, Wasps, Bees, various 

 Lepidoptera of which Bombyx is an example, in certain free-living 

 Nematode worms, Rotifers, and Gasterotricha, the faculty has 

 been re-acquired. 1 The male and female characteristics of the 

 sexual cells stand out clearly from their comparison in the Animal 

 and Plant Kingdoms. The male elements, as we have seen, 

 are produced by cells that multiply rapidly by division, and 

 are incapable of accumulating reserve material. Hence they 

 remain small, and their activity which is mainly of a mechanical 

 order, is spent in the rapid movements of vibratile cilia 

 or flagella. These are the antherozoids of the vegetable 

 Cryptogams and the spermatozoa of animals. The female 

 elements, on the contrary, are produced by cells in which 

 division is retarded, especially in the last stages of their 

 evolution, and of which the activity is essentially of a chemical 

 order directed chiefly towards the elaboration of the reserve 

 substances which accumulate in their protoplasm and increase 

 their volume, making the cell heavier and suppressing all 

 possibility of movement. 



These characters, with which even unicellular beings 

 are invested in order to reproduce themselves, before becoming 

 a part of an organism, are retained by all living forms, 

 and in each animal species the same individuals are capable 

 generally of producing only one of the two types of sexual 

 cells. This is especially marked among species which stand at 

 the beginning of a series and therefore live in the sea. It would 



1 We are not speaking here of artificial parthenogenesis, which is a 

 phenomenon requiring a special study. 



