DEVELOPMENT. 209 
body and tail; if the tail is cut off, it will add a body 
and head. Certain Worms may be cut into several pieces, 
and each part will regain what is needed to complete the 
mangled organism. The Star-fish can reproduce its arms; 
the Holothuria, its stomach; the Snail, its tentacles; the 
Lobster, its claws; the Spider, its legs; the Fish, its fins; 
and the Lizard, its tail. Nature makes no mistake by put- 
ting on a leg where a tail belongs, or joining an immature 
limb to an adult animal.’ In Birds and Mammals, the 
power is limited to the reproduction of certain tissues, as 
shown in the healing of wounds. Very rarely an entire 
human bone, removed by disease or surgery, has been re- 
stored. The nails and hair continue to grow in extreme 
old age. 
4. Likeness and Variation. 
It is a great law of reproduction that all animals tend 
to resemble their parents. A member of one class never 
produces a member of another class. The likeness is very 
accurate as to general structure and form. But it does 
not descend to every individnal feature and trait. In 
other words, the tendency to repetition is qualified by a 
tendency to variation. Like produces like, but not exact- 
ly. The similarity never amounts to identity. So that 
we have two opposing forces—the hereditary tendency to 
copy the original stock, and a distinct tendency to devi- 
ate from it. 
This is one of the most universal facts in nature. Ey- 
ery development ends in diversity. Every body knows 
that no two individuals of a family, human or brute, are 
absolutely alike. There are always individual differences 
by which they can be distinguished. Evidently a parent 
does not project precisely the same line of influences upon 
each of its offspring. 
This variability makes possible an indefinite modifica- 
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