To illustrate the difference between the two theories, we will make use of a giraffe. The opposers of natural selection state that in order to eat the leaves high up in the trees, these once smaller necked animals had to stretch their necks to reach their meal. As a direct result, their offspring are then born with longer necks. This continues in subsequent generations until we have the very long-necked giraffes we have today. On the other hand, Darwin’s theory suggests that the giraffe’s ancestors once had short necks. Mutations arose that caused long necks to occur. Since these long-necked individuals were able to reach leaves higher in the trees, they are able to eat more and become sturdier than their short-necked relatives. Thus, they produced more offspring and the long-neck mutation gradually spread. In other words, Darwin’s theory of natural selection had the notion of “survival of the fittest”.

Opposers of natural selection argue that for Darwin’s theory to work, a vast amount of time would be required for such an evolution to occur. While Charles Lyell’s estimation on the age of the earth seemed sufficient enough for such an evolution to occur, physicist Lord Kelvin’s estimated age of earth using its current temperature and rate of cooling came up with an age that did not allow enough time for natural selection to happen.

Other reasons are tied to religion, economics, and a bit of political ideology. Some opposers were not scientists but capitalists who found that the Lamarckian evolution fit more into their way of business rather than Darwin’s natural selection.