HAECKEL: HIS LIFE, WORK, AND COMPANIONS 88 
barely filled thirty-five pages. But a longer sketch soon 
followed ; and in 1856, after more than nineteen years 
labour, he yielded to the advice of Lyell and began to write 
out his views ‘‘ pretty fully.’’ Darwin’s collection of facts 
esiablished beyond cavil that organisms of the same species 
differ, no one being exactly like another; that these 
various characteristics tend to go down from generation to 
generation ; and that more organisms come into the world 
than there is room for, hence the struggle for life which 
leads to survival of the fittest. 
oe 
Of the particulars leading up to these conclusions 
regarding variation, heredity, and survival in living organ- 
isms, all are interesting, many are curious, and one or two 
should be mentioned. Darwin found that the elephant, 
though living nearly a hundred years, is the least prolific 
of animals, having an average of only six young. Yet if 
that ratio of increase in succeeding generations went on 
without casualties for 750 years, at the end of that period 
there would be eighteen millions of elephants living. In 
the case of flies : the house-fly lays batches of eggs having 
about 130 eggs in a batch; and has five or six batches in a 
season. In three weeks an egg becomes a fly, itself laying 
eggs. If all these lived, and half were females, the progeny 
in one season of a single female fly would be a million cubic 
feet of flies, reckoning 200,000 flies to a cubic foot. But 
in fact house-flies vary but little in number from year to 
year. Themass perish. Among plauts, the hedge-mustard, 
sisymbriuni sophia, often has in a single plant three- 
quarters of a million seeds. If these, and their products, 
at the same rate of increase, grew to maturity for three 
years, the land surface of the globe would not hold them 
all. But the mass perish by the way-side; only a few of 
the fittest survive. 
Hence arose the conclusion that from such selective 
action of external conditions come permanent varieties of 
plants and animals ; and the same causes in long periods of 
