CHAPTER XVI 



LIFE-HISTORIES 



1. The curve of life 2. Larval periods 3. Prolonged embryonic 

 life 4. Intricate life-histories 5. Prolonged youth 6. Ado 



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lescence 7. Senescence and rejuvenescence 8. Death 9. Re 

 trospect. 



1. The Curve of Life. One of the most significant char- 

 acteristics of living creatures is their " cyclical develop- 

 ment." The life-span of one is counted in days and of 

 another in months ; we reckon our own in years and the 

 Sequoia's in centuries, but there is for most an ascending 

 curve from the vita minima of the egg-cell (which often 

 dies in a few hours if it be not fertilised) to the vita maxima 

 of full strength, and thence a descending curve to the vita 

 minima of the outworn creature if the conditions of life 

 admit of senescence. The animal ovum divides and re- 

 divides, and there is built up an embryo. This may 

 develop steadily and directly into the likeness of its kind, 

 as in the case of reptile, bird, and mammal ; or it may 

 give rise to a quite divergent phase a larva of some sort 

 -such as caterpillar or tadpole, which by and by under- 

 goes metamorphosis and gets shunted on to the direct 

 line of development. Through more or less critical 

 phases of adolescence it becomes adult. It is a not in- 

 frequent achievement to lengthen out the period of 

 mature strength, but sooner or later the edifice begins to 

 crumble. "And so, from hour to hour, we ripe and ripe, 

 and then, from hour to hour, we rot and rot, and thereby 

 hangs a tale." 



In life-histories in general we have before us the 

 spectacle of a gradual movement towards a full epiphany, 

 but the details are extraordinarilv diverse. Life-histories 



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