INTR ODUC TION 9 



their common ancestor. Such is, in fact, the case in every plant and 

 animal whose development is accurately known. The first step in 

 development consists in the division of the ^g^ into two parts, each 

 of which is a cell, like the Q.gg itself. The two then divide in turn to 

 form four, eight, sixteen, and so on in more or less regular progres- 

 sion (Fig. 3) until step by step the ^fgg has spHt up into the multitude 

 of cells which build the body of the embryo, and finally of the adult. 

 This process, known as the cleavage or segmentation of the egg, 

 was observed long before its meaning was understood. It seems to 

 have been first definitely described in the case of the frog's egg, by 

 Prevost and Dumas (1824), though earlier observers had seen it; but 

 at this time neither the &gg nor its descendants were known to be 

 cells, and its true meaning was first clearly perceived by Bergmann, 

 Kolliker, Reichert, von Baer, and Remak, some twenty years later. 

 The interpretation of cleavage as a process of cell-division was fol- 

 lowed by the demonstration that cell-division does not begin with 

 cleavage, but can be traced back into the foregoing generation ; for the 

 egg-cell, as well as the sperm-cell, arises by the division of a cell pre- 

 existing in the parent-body. It is therefore deidved by direct descent 

 from an egg-cell of the foregoing generation, and so on ad infimtnm. 

 Embryologists thus arrived at the conception so vividly set forth by 

 Virchow in 1858 ^ of an uninterrupted series of cell-divisions extend- 

 ing backw^ard from existing plants and animals to that remote and 

 unknown period when vital organization assumed its present form. 

 Life is a continuous stream. The death of the individual involves no 

 breach of continuity in the series of cell-divisions by which the life 

 of the race flows onwards. The individual body dies, it is true, but 

 the germ-cells live on, carrying with them, as it were, the traditions 

 of the race from which they have sprung, and handing them on to 

 their descendants. 



These facts clearly define the problems of heredity and variation 

 as they confront the investigator of the present day. All theories of 

 evolution take as fundamental postulates the facts of variation and 

 heredity ; for it is by variation that new characters arise and by 

 heredity that they are perpetuated. Darwin recognized two kinds of 

 variation, both of which, being inherited and maintained through the 

 conserving action of natural selection, might give rise to a permanent 

 transformation of species. The first of these includes congenital or 

 inborn variations ; i.e. such as appear at birth or are developed 

 "spontaneously," without discoverable connection with the activities 

 of the organism itself or the direct effect of the environment upon it. 

 In a second class of variations are placed the so-called acquired char- 



1 See tlie quotation from the original edition of the Celliilarpatliologie at the head of 

 Chapter II., p. 45. 



