WASTE AND REPAIR. 223 



Is it of the same order as the ability of an injured crystal to 

 re-complete itself. In either case new matter is so deposited 

 as to restore the original outline. And if in the case of the 

 crystal we say that the whole aggregate exerts over its parts a 

 force which constrains the newly-integrated molecules to take- 

 a certain definite form, we seem obliged, in the case of the 

 organism, to assume an analogous force. If when the leg of 

 a lizard has been amputated there presently buds out the 

 germ of a new one, which, passing through phases of develop- 

 ment like those of the original leg, eventually assumes a like 

 shape and structure, we assert only what we see, when we 

 assert that the entire organism, or the adjacent part of it, 

 exercises such power over the forming limb as makes it a 

 repetition of its predecessor. If a leg is reproduced, where 

 there was a leg, and a tail where there was a tail, there 

 seems no alternative but to conclude that the forces around 

 it control the formative processes going on in each part. 

 And on contemplating these facts in connexion with various 

 kindred ones, there is suggested the hypothesis, that the 

 form of each species of organism is determined by a pecu- 

 liarity in the constitution of its units that these have a 

 special structure in which they tend to arrange themselves; 

 just as have the simpler units of inorganic matter. Let us 

 glance at the evidences which more especially thrust this 

 conclusion upon us. 



A fragment of a Begonia-leaf imbedded in fit soil and kept 

 at an appropriate temperature, Avill develop a young Begonia ; 

 and so small is the fragment which is thus capable of origin- 

 ating a complete plant, that something like a hundred 

 plants may be produced from a single leaf. The friend to 

 whom I owe this observation, tells me that various succulent 

 plants have like powers of multiplication. Illustrating a 

 similar power among animals, we have the often-cited experi- 

 ments of Trembley on the common polype. Each of the 

 four pieces into which one of these creatures was cut, grew 

 into a perfect individual. In each of these, again, bisection 



