3S4 PROVISIONAL HYPOTHESIS Chap. XXVII. 



that allied organic beings in the course of their metamorphoses 

 sometimes attain nearly the same structure after passing 

 through, widely different forms : or conversely, after passing- 

 through nearly the same early forms, arrive at widely different 

 mature forms. In these cases it is very difficult to accept 

 the common view that the first-formed cells or units possess 

 the inherent power, independently of any external agency, of 

 producing new structures wholly different in form, position, 

 and function. But all these cases become plain on the 

 hypothesis of pangenesis. The units, during each stage of 

 development, throw off gemmules, which, multiplying, are 

 transmitted to the offspring. In the offspring, as soon as 

 any particular cell or unit becomes partially developed, it 

 unites with (or, to speak metaphorically, is fertilised by; the 

 gemmule of the next succeeding cell, and so onwards. But 

 organisms have often been subjected to changed conditions 

 of life at a certain stage of their development, and in 

 consequence have been slightly modified ; and the gemmules 

 cast off from such modified parts will tend to reproduce parts 

 modified in the same manner. This process may be repeated 

 until the structure of the part becomes greatly changed at 

 one particular stage of development, but this will not ne- 

 cessarily affect other parts, whether previously or subsequently 

 formed. In this manner we can understand the remarkable 

 independence of structure in the successive metamorphoses, 

 and especially in the successive metageneses of many animals. 

 In the case, however, of diseases which supervene during old 

 subsequently to the ordinary period of procreation, and 

 which, nevertheless, are sometimes inherited, as occurs with 

 brain and heart complaints, we must suppose that the organs 

 were affected at an early age and threw off at this period 

 affected gemmules ; but that the affection became visible or 

 injurious only after the prolonged growth, in the strict sense 

 of the word, of the part. In all the changes of structure 

 which regularly supervene during old age, we probably see 

 the effects of deteriorated growth, and not of true develop- 

 ment. 



The principle of the independent formation of each part, 

 owing to the union of the proper gemmules with certain 



