Thoughts on tlie Hymenoptera Trichogrammatidae etc. 77 



the exceedingly close resemblance of North American and Australian 

 species of Trichogrammen and Oligosita and of species of other genera 

 common to the countries. But, more than this. To suppose an indepen- 

 dent origin forces upon us either one or two admittances which decide 

 the question negatively or affirmitively. Either we must admit mi- 

 gration of the ancestors or eise an entire independence of origin for each 

 country. To admit the former is to decide in the negative. To admit 

 an entire independence of origin involves a denial of the whole theory 

 of common descent and the affirmation that life arose in each country 

 independently and trevolved there without any relation to other 

 evolutions. But in order to admit this we must explain away a legion 

 of facts such as the striking resemblances of species from the two 

 regions, the occurrence of similar genera and other higher groups 

 not only with great structural and other resemblances but with all the 

 peculiarities of habit. The thought persists, however, that it is just 

 as possible for life to arise in many places at one time as it is for it to 

 arise only in one place 1 ) and if the possibility is admissable then it 

 is quite conceivable that, the fundamental conditions of life being the 

 same in all places, the same evolutionary produetions resulted. The 

 conditions for life are practically iventical all over the earth just as 

 the various forms of life are united in that they are merely different 

 expressions of the same fundamental substance. It is conceivable, 

 therefore, that the same pind of evolution should take place in different 

 countries, producing much the same kind of life and often quite the 

 same. This is true for development in man, since the most ancient 

 and widely separated civilizations are found to have arisen quite 

 independently and yet are of the same kind, differing only in their 

 expression. But is not this fact also an argument for common descent? 

 Does it not also go to show that given the same traditions the same 

 response is given to the same Stimuli, the expressed response differing 

 only in its details? This same tradition certainly strongly implies 

 common origin. Although we can at first see no actual impossibility 

 that evolutions may have arisen independently, the facts are too 

 patently in favor of one common origin to entertain the other. The 

 whole experience of naturalists leads to its aeeeptance and experience 

 is the only criterion of reason. The produetions of no two separated 

 countries are the same which would not be expected were evolutions 

 of independent origin and parallel in produetions or nearly so; the 

 distribution of closely related species in one country is the same as 

 the distribution of similar species over the earth, one species gradually 

 fades out and its place is taken by another; distant countries in the 

 same latitude with nearly the same physiographical features and 

 climates have widely different fauna; the presence of almost impassible 

 barriers to migration is closely related to the presence of distinet 



*) Some naturalists think it extremely likely that primitive life arises con- 

 tinually and is still doing so, whenever the conditions become favorable. After 

 seeing the evidence, I am of the same opinion. 



l.Heft 



