68 PROBLEMS OF GENETICS 



terminal. 2 So also with variation in the number of ribs, when a 

 lumbar vertebra varies homoeotically into the likeness of the 

 last dorsal and bears a rib, the rib placed next in front of this, 

 which in the normal trunk is the last, shows a definite increase 

 in development. 



The consequences of such homoeoses are sometimes very- 

 extensive, involving readjustments of differentiation affecting 

 a long series of members, as may easily be seen by comparing 

 the vertebral columns of several individual Sloths 3 (whether 

 Bradypus or Choloepus) to take a specially striking example. 



It may be urged that no feature as yet enables us to perceive 

 wherein lies the primary distinction which determines such 

 variation, whether it is due to a difference in the dividing forces 

 or in the material to be divided. If for instance we were to 

 imitate such a series of segments by pressing hanging drops of 

 a viscous fluid out of a paint- tube by successive squeezes, the 

 number of times the tube is contracted before it is empty will give 

 the number of the segments, but their size may depend either 

 on the force of the contractions or on the capacity of the tube, 

 or on various other factors. Nevertheless in the case of the 

 variation of terminal members, whatever be the nature of the 

 rhythmical impulse which produces the series of organs, the ele- 

 vation of the normally terminal member in correspondence with 

 the addition of another is what we should expect. 



If the organism acquired its full size first and the delimitation 

 of the parts took place afterwards, there might be some hope that 

 the resemblance between living patterns and those mechanically 

 caused by wave-motion might be shown to be a consequence of 

 some real similarity of causation, but in view of the part played 

 by growth, appeal to these mechanical phenomena cannot be 

 declared to have more than illustrative value. Similarly in as 

 much as living patterns appear, and almost certainly do in reality 

 come into existence by a rhythmical process, comparisons of 

 these patterns with those developed in crystalline structures, and 

 in the various fields of force are, as it seems to me, inadmissible, 

 or at least inappropriate. 



2 Materials for the Study of Variation, No. 249, p. 217; and p. 272. 



3 Materials, p. 118. 



