Présentation de l'éditeur
Acceleration of Development in Fossil Cephalopoda JAMES PERRIN SMITH. I deal Recapitulation in Progressive Forms. IN THE development of organisms there are two theoretical extremes, the one with simple persistence without modification, the other with complete modification. The former is almost realized in the Protozoa, the latter is approached by the higher vertebrates. All other organisms, in their development, fall somewhere between the two extremes, coming into being in simpler form, and becoming more complex in the course of life. Each starts out on somewhat the same plane of development as its distant ancestors, inheriting potentially all the characters of all its ancestors, tending to take on some characters that its ancestors never had, and to transmit the old and the new to its own posterity. Theoretically, each organism ought to recapitulate all its race history, each stage of growth corresponding in character and in size to successive ancestral forms. This is true, in a general way, in some groups, for most later members of genetic series have increased in size with increased complexity of development. FIG. 1. This is partly true even of the highly specialized Cephalopods, for there is a constant tendency to increase in size from the simple Goniatites of theD evonian to the complex Ammonites of the Jurassic. The increase in size accompanying the addition of ontogenic stages is especially striking in a primitive genetic series of genera near each other in time, and relatively near the beginning of the race, as in the lineage of GoniatitesG astrioceras-C olumbites.
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