DEVELOPMENT. 215 



continually growing; the long bones of higher animals 

 increase in length so long as the ends (epiphyses) are sep- 

 arate from the shaft. The limbs of Man, after birth, 

 grow more rapidly than the trunk. 



The power of regenerating lost parts is greatest where 

 the organization is lowest, and while the animal is in the 

 young or larval state. It is really a process of budding. 

 ■The upper part of the Hydra, if separated, will reproduce 

 the rest of the body; if the lower part is cut off, it will 

 add the rest. Certain Worms may be cut into several 

 pieces, and each part will regain what is needed to com- 

 plete the mangled organism. The Star-tish can reproduce 

 its arms; the Holothurian, its stomach ; the Snail, its ten- 

 tacles ; 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 belongs, or join- 

 ing an immature limb to an adult animal."" In Birds and 

 Mammals, the power is limited to the reproduction of cer- 

 tain tissues, as shown in the healing of wounds. Very 

 rarely an entire human bone, removed by disease or sur- 

 gery, 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 theif parents. A member of one class never 

 produces a member of another class. Tlic likeness is very 

 accurate as to general structui'e 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 ex- 

 actly. The similarit}' never amounts to identity. So that 

 we have two opposing tendencies — the hereditary ten- 

 dency to copy the original stock, and a distinct tendency 

 to deviate from it. 



