44 progress: its law and cause. 



and so on continually : oacli organ as it is developed ser- 

 ving, by its actions and reactions upon the rest, to initiate 

 new complexities. The first pulsations of the fetal heart 

 must simultaneously aid the unfolding of every part. The 

 growth of each tissue, by taking from the blood special 

 proportions of elements, must modify the constitution of 

 the blood ; and so must modify the nutrition of all the 

 other tissues. The heart's action, implying as it does a cer- 

 tain waste, necessitates an addition to the blood of effete 

 matters, which must influence the rest of the system, and 

 perhaps, as some think, cause the formation of excretory 

 organs. The nervous connections estabhshed among the 

 viscera must further multiply their mutual influences : and 

 so continually. 



Still stronger becomes the probability of this view when 

 we call to mind the fact, that the same germ may be 

 evolved into different forms according to circumstances. 

 Thus, during its earlier stages, every embryo is sexless — 

 becomes either male or female as the balance of forces act- 

 ing upon it determines. Again, it is a well-established fact 

 that the larva of a working-bee will develop mto a queen- 

 bee, if, before it is too late, its food be changed to that on 

 which tlie larv?g of queen-bees are fed. Even more remark- 

 able is the case of certain entozoa. The ovum of a tape- 

 worm, getting into its natural habitat, the intestine, unfolds 

 into the well-knowTi form of its parent ; but if carried, as 

 it frequently is, into other parts of the system, it becomes 

 a sac-like creature, called by naturalists the JEchinococcus 

 — a creature so extremely different from the tape- worm in 

 aspect and structure, that only after careful investigations 

 has it been proved to have the same origin. All which 

 instances imply that each advance in embrj^onic complica- 

 tion results from the action of incident forces upon the 

 complication previously existing. 



Indeed, we may find d, 2?non reason to think that the 



