THE CONTINUITY OF LIFE 21 



In the above exposition of phylogenesis there can be seen 

 at once both its advantages and its disadvantages as an his- 

 torical record. 



In cases in which a line of descent is well represented by a 

 series of adult animals, the advantage of being able to study 

 large forms with functional parts is obvious; but where the 

 extinction of intermediate forms has obliterated the record 

 at some important point, the phylogenetic data fail completely 

 and must be supplied by the parallel history found in the indi- 

 vidual development of the nearest allied forms. The great- 

 est assistance has often been furnished by palaeontology, but 

 as the hard parts alone leave their imprint in the rocks, they 

 are of little or no assistance in the history of many of the sys- 

 tems. Again, through the metamorphosis of the earlier geo- 

 logical formations and the consequent obliteration of all 

 organic remains occurring in them, the palaeontological record 

 has lost beyond hope of recall all of its early stages, and at the 

 period of the first fossiliferous strata, the main classes of 

 animals as we have them at present, had already become 

 established. 



It is here that the study of comparative embryology lends 

 its assistance, since in the embryological record the earliest 

 stages are preserved, although often overlaid with secondary 

 modifications. By its aid may be traced, not only the lines 

 connecting any two forms (Rule II. above), but it furnishes 

 faint though definite clews to the early history of animal de- 

 velopment previous to the beginning of the palseontological 

 record. Its defects, though many, are not the same as those 

 of the phylogenetic record, and the two thus reinforce one 

 another to a remarkable degree, each completing the gaps left 

 in the other, and corresponding closely in those places in 

 which both records are preserved. 



The exposition of developmental history, or ontogenesis, 

 may be given in the form of laws as in the former case. 



I. The developmental history of an animal includes all 

 stages from that of the fertilized egg (ovum) to that of the 

 sexually mature adult, and is not in any way interrupted by 



