496 AN AMERICAN TEXT- HOOK OF PHYSIOLOGY. 



both civilized ami savage races, is rendered manifest in the offspring that 

 results from the union of the two. Reversionary characters are often more 

 prominent during youth than during "later life — a fact that has been quoted in 

 favor of their explanation on the theory of latency. 



Regt in ration. — The farts of regeneration of lost parts must also be taken 

 into account in a theory of heredity. Such regeneration may be either physi- 

 ological or pathological. Physiological or normal regeneration has reference 

 to the reproduction of parts that takes place during the normal life of the 

 individual, such as the constant growth of the deeper layers of the epidermis 

 to replace the outer layers that are as constantly being -lied. I'atho/or/ical 

 regeneration refers to the replacement of parts lost by accident, and presents 

 the more interesting and striking examples. The power of pathological 

 regeneration in man and the higher mammals is limited. A denuded surface 

 may be re-covered with epithelium ; the central end of a cut nerve may grow 

 anew to its termination ; the parts of a broken bone may reunite; muscle may 



reappear; connective-tissue, blood-corpuscles, and bl 1-vessels may develop 



readily; and in the healing of every wound a regeneration of parts takes 

 place. But in descending the scale of animal life the regenerative power 

 becomes progressively stronger, and in many plants and low animals it is 

 marvellous. Thus, the newt may replace a lost leg, the crab a lost claw, the 

 snail an eyestalk and eye. If an earth-worm be cut in two, one half may 

 regenerate a new half, complete in all respects. A hydra may be chopped 

 into fragments and each fragment may re-grow into a complete hydra. From 

 a small piece of the leaf of a begonia, planted in moist earth, a new plant 

 with all its parts may arise. It is evident that the existing parts of an organ- 

 ism, if not too specialized, possess the power of restoring parts that are lost ; 

 under ordinary circumstances this power is latent. The growth of tumors is 

 perhaps allied in nature to regeneration. A study of regeneration shows that 

 in many cases the process of building anew follow - the same course as the 

 original embryonic growth. It is properly a phenomenon of hereditv. 



The Inheritance of Acquired Characters. — No topic in heredity has been 

 more debated during the past twenty years than that of the possibilitv of the 

 transmission to the offspring of characteristics that are acquired by the parents 

 previous to the discharge of the germ-cells, or, in the case of the mammalian 

 female, previous t<> parturition. Obviously, no one denies this possibility in 

 the unicellular organisms, where reproduction by fission prevails, for there the 

 protoplasm of the body of one parent becomes the substance of two offspring; 

 in the transformation nothing is lost, and hence whatever peculiarities the ances- 

 tral protoplasm has acquired are transferred bodily to the descendants. But 

 in multicellular forms, where sexual reproduction exists, the case is very dif- 

 ferent, for here whatever i- transmitted is transmitted through germinal cells, 

 or germ-plasm, a- the hereditary substance contained in the germ-cells is now 



commonly called. The problem then resolves itself into that of the relation 

 of tin' germ-plasm to the protoplasm of the rest of the body, the so-called 

 somatoplasm; and the question to he answered is this: Are variations in the 



