PREFACE. 



The objects of the present work are, firstly, to determine the 

 mutual relations of those diversified forms Avhich the recent 

 researches of i^aturalists have sho^Ti to be propagated from 

 each other, in many different species among the lower orders 

 of both kingdoms of I^ature ; and, secondly, to consider 

 what analogy can be traced between the successive forms in 

 these species, and the phenomena which occur in the repro- 

 duction of those higher in the scale of organization, where, 

 to all appearance, like always produces like. 



The Author is well aware that a much abler pen than his 

 would have but little chance of enlisting the interest of any 

 large circle of readers in such a work as the present ; for, 

 though it would not be easy to point to any subject in the 

 whole range of Biological Science more calculated to excite 

 feelings of wonder and enquiry, yet the obscurity of some 

 of the questions iuA'-olved, while it has stimulated the 

 zeal of many earnest investigators, has no doubt kept back 

 others from giving the same attention to this, as to other 

 branches of Physiology ; besides which, it is not to be denied 



