Animals change and acquire certain characteristics based on their desires. If they want a feature badly enough they will develop it!
Your theory is pretty good for your time, however there is some information that you haven't taken into consideration.
Change in an organisms features is dependant on their survival rates and the opposite sex preference. They obtain the characteristics through generations, not as individuals
Overheard you talking about evolution! I agree with you in most parts Wallace. Change in animal features happens over time, through generations, it is not only affected by the population of the species, but also environmental factors like regional temperatures and diverse food sources.