THE METHOD OF CEEATION OF ORGANTC FORMS. 185 



los and cetaceans, increasing as they have done from a simpler 

 commencement, present examples of repetitiye acceleration of 

 growth-force. 



e. Complication of a single element of repetition is accomplished 

 apparently by a double repetition. This is best understood by the 

 consideration of the transition from simple to complex teeth. In 

 the cetaceans this occurs in the Squalodonts ; the cylindric incisors 

 are followed by flattened ones, then by others grooved on the fang, 

 and then by two-rooted, but never double-crowned teeth. This is 

 the result of anteroposterior repetitive acceleration of the simple 

 cylindric dental type of the ordinary toothed cetacean. 



Another mode of dental complication is by lateral repetition. 

 Thus, the heel of the sectorial tooth of a Carnivore is supported 

 by a fang alongside of the usual posterior support of a premolar, 

 and is the result of a repetitive effort of growth-force in a transverse 

 direction. More complex teeth, as the tubercular molars, merely 

 exhibit an additional lateral repetition, and sometimes additional 

 longitudinal ones. As is well known, the four tubercles of the 

 human molar commence as similar separate knobs on the dental 

 papilla. 



The above are cited as examples to explain the meaning of the 

 proposition. When fuller demonstration is desired, a greater num- 

 ber might be given. 



• 



B. On Cell-Repetition. 



That each additional act of creation in growth was originally 

 identical with one which preceded it, and therefore an exact repe- 

 tition in its character and results, is proved by the following con- 

 siderations. 



It has been already determined by the study of homologies that 

 all organs and parts of an organism can be referred to an original 

 simple archetype. 



The question then remains as to whether the first element, or 

 lowest term, of a given organized part is essentially a new struct- 

 ure, or whether it be a repetition of some previously existing one. 

 It may be asserted that the simplest expressions which shall cover 

 all organs are the solid segment, and the hollow sack, and tube. 

 For example, we have already noted that the ultimate element of 

 the limb is the first segment of the single ray of Lepidosiren. Is 

 this short cartilaginous cylinder (which probably represents the 

 fore limb of some undiscovered member of the Dipnoi) a result of 



