ORIGIN AND EVOLUTION OF LIFE 509
quickly darting types of nnanaored fishes. The double pointed, fuai- fonu body, in wbidi the segmented propelling muscles are external and a stiffening notochord is central, is the fish prototype, which more or less clearly survives in the existing lancelets (Amphtoxus) and in the larval stages of the degenerate Ascidians. These animals furnish nu- merous embryonic and larval proofs of descent from nobler types.
Taking the whole history of vertebrate life from the beginning, we observe that every prolonged adaptive phase in a similar habitat be- comes impressed ia the hereditary charactors of the chromatin, which throughout the development of new adaptive phases always retains more or less potentiality of repeating the embryonic, immature, and even the mature structures of older adaptive phases of older environ- ments. The chromatin is at once the most conservative and the most progressive center of physico-chemical evolution ; it records past adapta- tions, it meets the emergencies of the present through the adaptability which it imparts to the organism in its distribution throughout every living cell; it is continuously giving rise to new characters and fnnc- tioDB. This law of ancestral repetition, formulated by Louis Agassiz
��FlO. 8. PG&IOD OP TUB GlBLT AFPBABJLtlCE OF TBBUBaTUIAL Is VEBISBaATBS IHD
VBBTEBBAiEa. Pal«oeeoBra[)h; at the eartb in earlj Lower Deraolan time, sbowlnf the hypathetlcal southera continent Oondicana and the Euraaiatlc Inlaad sea Ttthtii, according to the hypotheseB ot Sueaa. After ScbDcbert, 1916.
and developed by Haeekel and Hyatt, dominated biological thought during thirty years of the nineteenth century (1865-1895), and with more or less success a highly speculative solution of the ancestral his- tory of the vertebrates was sought in embryonic development and com-
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