DISEASES OF THE GENERATIVE OBGANS. 181 



(9) From the doubling of parts or of the entire body — double 

 monsters, douljled heads, doubled bodies, extra limbs, etc. — redundant 

 development. (PL XTX, figs. 1, 2, 3.) 



Causes. — The causes of monstrosities are varied. Some, like extra 

 digits, lack of horns, etc., run in families, which produce them with 

 absolute certainty when bred in the direct line, although they were 

 originally acquired peculiarities which have merely been fixed by 

 long habit in successive generations. The earliest horse had five toes, 

 and even the most i-ecent fossil horse had three toes, of Avhich the 

 two lateral ones are still represented in the modern animal by the two 

 splint bones. Yet if our horse develops an extra toe it is pronounced 

 a monstrosity. A more genuine monstrosity is the solid-hoofed pig, 

 in which two toes have been merged into one. Another of the same 

 kind is the solid shank bone of the ox, which consists of two bones 

 united into one, but which are still found apart in the early fetus. 

 Though originally acquired peculiarities, they now breed as invari- 

 ably as color or form. 



Other monstrosities seem to have begim in too close breeding, by 

 which the powers of symmetrical development are impaired, just as 

 the procreative power weakens under continuous breeding from the 

 closest blood relations. A monstrositv consisting in the absence of 

 an organ often depends on a simple lack of development, the result 

 of disease or injury, as a young bone is permanently shortened by 

 being broken across the soft part between the shaft and the end, the 

 only part where increase in length can take place. As the result of 

 the injury the soft, growing layer becomes prematurely hard and 

 all increase in length at that end of the bone ceases. This will ac- 

 count for some cases of absence of eye, limb, or other organ. 



Sometimes a monstrosity is owing to the inclosure of one ovum in 

 another while the latter is still but a soft mass of cells and can easily 

 close around the first. Here each ovum has an independent life; 

 they develop simultaneously, only the outer one having direct con- 

 nection with the womb and being furnished with abundant nourish- 

 ment advances most i-apidly and perfectly, while the inclosed and 

 starved ovum is dwarfed and imperfect often to the last degree. 



In many cases of excess of parts the extra part or member is mani- 

 festly derived from the same ovum, and even the same part of the 

 ovum, being merely the effect of a redundancy and vagary of growth. 

 Such cases include most instances of extra digits or other organs, and 

 even of double monsters, as manifested by the fact that such extra 

 organs grow from the normal identical organs. Hence the extra 

 digit is attached to the normal digit, the extra head to the one neck, 

 the extra tail to the croup, extra teeth to the existing teeth, and 

 even two similarly formed bodies are attached by some point com- 

 mon to both, as the navels, breastbones, back, etc. (PI. XIX, figs. 



