E-text prepared by Michael Ciesielski, Cori Samuel, and the Project Gutenberg Online Distributed Proofreading Team

Probably the most important years in anyone's life are those eight orten preceding the twenty-first birthday. During these yearsHeredity, one of the two great developmental factors, bears itscrop, and the seeds sown before birth and during childhood come tomaturity. During these years also the other great developmental forceknown as Environment has full play, the still plastic nature ismoulded by circumstances, and the influence of these two forces isseen in the manner of individual that results.
This time is generally alluded to under two heads: (1) Puberty, (2)Adolescence.
By Puberty we understand the period when the reproductive organs aredeveloped, the boy or girl ceasing to be the neutral child andacquiring the distinctive characteristics of man or woman. The actualseason of puberty varies in different individuals from the eleventh tothe sixteenth year, and although the changes during this time are notsudden, they are comparatively rapid.
By Adolescence we understand the time during which the individual isapproximating to the adult type, puberty having been alreadyaccomplished. Adolescence corresponds to the latter half of thedevelopmental period, and may be prolonged even up to twenty-fiveyears.