Hence, as in the case of corporeal structures, we ought to find in nature, not the actual transitional gradations by which each complex instinct has been acquired-for these could be found only in the lineal ancestors of each species-but we ought to find in the collateral lines of descent some evidence of such gradations; or we ought at least to be able to show that gradations of some kind are possible; and this we certainly can do.
edo nolabaiteko mailaketak izan litezkeela frogatzeko modua izan beharko genuke bederen, eta hau, zalantzarik gabe, egin ere, egin dezakegu.