Long before having arrived at this part of my work, a crowd of difficulties will have occurred to the reader. Some of them are so grave that to this day I can never reflect on them without being staggered; but, to the best of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal to my theory.

These difficulties and objections may be classed under the following heads:-Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined?

Secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modification of some animal with wholly different habits? Can we believe that natural selection could produce, on the one hand, organs of trifling importance, such as the tail of a giraffe, which serves as a fly-flapper, and, on the other hand, organs of such wonderful structure, as the eye, of which we hardly as yet fully understand the inimitable perfection?

Thirdly, can instincts be acquired and modified through natural selection? What shall we say to so marvellous an instinct as that which leads the bee to make cells, which have practically anticipated the discoveries of profound mathematicians?

Fourthly, how can we account for species, when crossed, being sterile and producing sterile offspring, whereas, when varieties are crossed, their fertility is unimpaired?

Darwin såg ett antal problem med sin teori. Han säger att han fortfarande får frossa bara han tänker på dem.

1. Varför finns inte otaliga mellanformer, varför finns arterna väl begränsade?

2. Hur skulle t.ex. fladdermöss formats av helt annorlunda djur? Varför har giraffen en svans för flugors skull? Den är ju inte livsviktig. Hur har ögat blivit perfekt?

3. Hur kunde djuren få sina instinkter genom urval? Bina är födda till byggmästare.

4.Varför är det så svårt för olika arter att få gemensam avkomma, då det inte är problem mellan djurvariationer av samma art?

Darwin betraktade häst och åsna som olika arter, trots att båda är hästdjur.

De kan få en steril avkomma, medan häst och ko kan det aldrig!