CHAPTER XIX 
CHANGES IN THE LIFE-CYCLE AND CHANGES IN THE 
ENVIRONMENT 
The Life Histories of Some Animals 
In the life histories of many animals and plants a series of 
stages succeed each other until, with the completion of the cycle, 
the starting point is reached again. In some species the series 
of forms that make up the life-cycle seem to change without any 
change taking place in the environment, that remains the same 
throughout, but in other cases the change in the cycle is asso- 
ciated with a change in the environment. It is this latter class 
that we may now examine, although it will also be profitable not 
to neglect the other. 
The life-cycle of most animals is made up of a single adult 
individual and a series of embryonic stages through which the 
embryo passes to become an adult. The cycle consists, therefore, 
of adult, egg (and sperm), embryo, larva, adult. In other cases 
the life-cycle is more complex, as it may contain more than 
one adult form and include several modes of reproduction. 
In some species the cycle seems to complete itself under con- 
stant external conditions, as stated above, while in closely related 
groups the change appears to be connected with a change in the 
environment. For instance, many of the marine hydroids show 
an alternation of generations that does not appear to be directly 
connected with changes in their surroundings. The egg develops 
into a swimming embryo that settles down and becomes a polyp. 
The polyp produces buds, all of which are also polyps. This 
process continues until the colony has reached a certain stage, 
when a new kind of bud arises that becomes a jellyfish. The 
jellyfish detaches itself, and comes to lead an independent exist- 
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