THE ORIGIN ^ND DEVELOPMENT OF ORGANISMS 329 



tion that our present understanding gives us an intelligent 

 grasp of the law of conformity to type. Future experiments 

 and discussions may modify our present ideas in many details: 

 but it is practically certain that the fundamental law, in ac- 

 cordance with which successive generations tend to resemble 

 one another, is now so well understood that it is not likely to 

 be changed greatly by subsequent investigation. The prin- 

 ciple underlying this conception of Weismann's is spoken of 

 as that of the continuity of germ plasm. A brief resume of 

 the theory is as follows : 



Reproduction by Simple Division. It is not difficult to 

 understand that when an animal multiplies by simple division, 

 the offspring will be similar to each other. When, for example, 

 an Amoeba divides, it would be almost impossible to see how 

 the two parts should be otherwise than alike, since they are 

 each half of the same individual. So, too, when yeast multi- 

 plies by budding, it is not difficult to understand that the buds 

 which grow from the side of the older cell will be like the old 

 cell. If a cell is thus capable of dividing, it would be very 

 difficult to see how the bud could in any degree be unlike its 

 parent, except as it may be changed by future conditions. 

 So, too, with those multicellular animals and plants that mul- 

 tiply by the process of budding, the conformity to type is nat- 

 ural rather than marvelous. When Hydra (Fig. 69) pro- 

 duces a small bud on its side, which grows to the size of the 

 original and then breaks away, it is not difficult to see why 

 the bud should be like the parent, for it would be difficult to 

 understand how it could be otherwise. So, too, when a branch 

 of a tree is broken off, takes root, and grows into a new tree 

 when placed in the ground, it would be difficult to understand 

 why the new individual should be different from the parent 

 tree, since it is really a part of it. In all of these cases the 

 conformity to type is natural and presents no special puzzle, 

 beyond the fact that animals and plants have the power 

 of dividing and reproducing at all. Conformity to type in 



