504 EPOCHS OF DIFFEKENTIATIOX. 



series, such a perfect uniformity in the condition of temperature obtain- 

 ed, the same is often observed in the first periods of individual develop- 

 ment. The circumstances under which the ovum commences its career, 

 even in the highest tribes, insure for it a perfect relief from every varia- 

 tion of heat. Included in the body of the female, it is cut off from all 

 external causes of disturbance, and kept at the temperature of her body, 

 whatever that temperature may be. In those cases, as in birds, in which 

 the embryo is developed under circumstances of necessary exposure, a 

 strong instinct is called into operation, and, by the incubation of the pa- 

 rent, the necessary uniformity is secured. Again, in other instances, as 

 in the ova of insects, which, by reason of their minuteness and their fre- 

 quently exposed position, although they may run through their earlier 

 changes with relatively great rapidity, some accomplishing them in the 

 almost uniform warmth of a summer's day, development never does nor 

 can occur until the required condition, even if it be temporary, as to uni- 

 formity of temperature, is reached. 



These considerations, though not affording an absolute proof that the 

 career of development is guided by the influence of external physical 

 conditions, are sufficiently significant to cast an air of probability over 

 that doctrine ; and even if we adopt the view that the developing germ 

 possesses a plastic power, which spontaneously compels it to run forward 

 from stage to stage in a predestined career if we recall what has already- 

 been said respecting that plastic power, that perhaps it is itself nothing 

 more than a manifestation of the remains of antecedent physical impres- 

 sions, we are really brought back to the same starting-point ; and, under 

 any hypothesis, we encounter, sooner or later, as a necessary postulate, 

 the grand doctrine that, directly or indirectly, development is a function 

 of external physical condition. 



It is not to be supposed that differentiation takes place with equal 

 Epochs of dif- ease at a H periods of the history of organic forms, whether 

 ferentiation. we consider them in the great scale, as constituting the ani- 

 mal series, or* on the small, in the individual. There are undoubtedly 

 epochs in each of their histories at which the exertion of an external in- 

 fluence will produce an effect infinitely greater than that which would 

 occur at any other moment. If we may be permitted to use such a me- 

 chanical illustration, the career of an organism recalls the flight of a 

 heavy projectile, as a shell, thrown upward, which, at the first moments 

 of its ascent and the last of its descent, pursues its way irresistibly, but 

 when it is at the top of its flight, and the momentum which had been im- 

 parted to it is just ceasing, the slightest breath of air, or the exertion of 

 any other insignificant force, will divert it into a path different from that 

 in which it would have gone ; and so, in the career of an organism, there 

 are moments when forces, which, at another time, would have been unfelt, 



