This effort sometimes causes changes in its body...

This effort sometimes causes changes in its body, like the appearance of the web uniting the toes of the duck when it was forced by its environment to swim in order to look for its food in the lakes, or the lengthening of the neck of the giraffe when it was forced to make use of the branches of lofty trees. (b) Though these organic changes take place gradually over many generations, they later pass from parents to offspring.

(c) Among the living beings there is a severe struggle for the continuation of their life, for procuring food and for selecting a suitable mate. This struggle for existence, that is a clash with the factors of the environment of life…