500 ' CONCLUSION. 



classes various structures are formed on the same pattern, 

 and at a very early age the embryos closely resemble each 

 other. Therefore I cannot doubt that the theory of de- 

 scent with modification embraces all the members of the 

 same great class or kingdom. I believe that animals are 

 descended from at most only four or five progenitors, and 

 plants from an equal or lesser number. 



Analogy would lead me one step further, namely, to the 

 belief that all animals and plants are descended from some 

 one prototype. But analogy may be a deceitful guide. 

 Nevertheless all living things have much in common, in 

 their chemical composition, their cellular structure, their 

 laws of growth, and their liability to injurious influences. 

 We see this even in so trifling a fact as that the same 

 poison often similarly affects plants and animals; or that 

 the poison secreted by the gall-fly produces monstrous 

 growths on the wild rose or oak tree. With all organic 

 beings, excepting perhaps some of the very lowest, sexual 

 reproduction seems to be essentially similar. With all, as 

 far as is at present known, the germinal vesicle is the 

 same; so that all organisms start from a common origin. 

 If we look even to the two main divisions — namely, to the 

 animal and vegetable kingdoms — -certain low forms are so 

 far intermediate in character that naturalists have disputed 

 to which kingdom they should be referred. As Professor 

 Asa Gray has remarked, " the spores and other reproduc- 

 tive bodies of many of the lower algae may claim to have 

 first a characteristically animal, and then an unequivocally 

 vegetable existence." Therefore, on the principle of 

 natural selection with divergence of character, it does not 

 seem incredible that, from some such low and intermediate 

 form, both animals and plants may have been developed; 

 and, if we admit this, we must likewise admit that all the 

 organic beings which have ever lived on this earth may be 

 descended from some one primordial form. But this infer- 

 ence is chiefly grounded on analogy, and it is immaterial 

 whether or not it be accepted. No doubt it is possible, as 

 Mr. Gr. H. Lewes has urged, that at the flrst commence- 

 ment of life many different forms were evolved- but if so, 

 we may conclude that only a very few liave left modified 

 descendants. For, as I have recently remarked in regard 

 to the members of each great kingaom, such as the Verte- 



