DEVELOPMENT 427 



reproduce its arms ; the holothurian, 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 putting on a leg where a tail be- 

 longs, or joining an immature limb to an adult animal. 173 

 In birds and mammals, the power is limited to the repro- 

 duction of certain tissues, as shown in the healing of 

 wounds. Very rarely an entire human bone, removed 

 by disease or surgery, has been restored. 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 individual feature and trait. 

 In other words, the tendency to repetition is qualified 

 by a tendency to variation. Like produces like, but not 

 exactly. The similarity never amounts to identity. So 

 that we have two opposing tendencies the hereditary 

 tendency to copy the original stock, and a distinct ten- 

 dency to deviate from it. 



This is one of the most universal facts in nature. 

 Every development ends in diversity. All know that 

 no two individuals of a family, human or brute, are 

 absolutely alike. There are always individual differ- 

 ences by which they can be distinguished. Evidently a 

 parent does not project precisely the same line of influ- 

 ences upon each of its offspring. 



This variability makes possible an indefinite modifica- 

 tion of the forms of life. For the variation extends to 

 the whole being, even to every organ and mental char- 

 acteristic as well as to form and color. It is very slight 



