The case for transitionals is very simple:

What a transitional organism is ...

In the observance of life today we see transitions in form from one generation to the next, We can study such trends in detail and document each step along the way, with field studies and with laboratory experiments. What we see is this:

Organism {A} - - (evolutionary path) - - Organism {B} - - (evolutionary path) - - Organism {C}

This is normal everyday observed and verified evolution by the change in hereditary traits in populations from generation to generation.

When we compare traits between population {A}, {B} and {C} we see several things:

• We see change in populations over time.
• We see variation of traits in each population and
• We see that some traits are shared by population {A}, {B} and {C}
• We see that some traits are shared between population {A} and {B} but not with {C}
• We see that some traits are shared between population {B} and {C} but not with {A}
• We see that population {A} has some traits that neither {B} or {C} have
• We see that population {C} has some traits that neither {A} or {B} have
• We do NOT see traits shared between population {A} and {C} but not with {B}

What we see is that population {A} shares some traits with population {B} but not all traits, and we see that population {B} shares some traits with population (C) but not all traits.

What we see is that, of all the traits that population {A} and {C} do NOT share, we see that population {B} shares some of them with population {A} and some of them with population {C}.

In practice we also see that the most traits are shared by all three populations, and that the amount of change between populations outside the amount of variation within each population is small. This factor demonstrates homologies and hereditary relationships rather than analogies and convergent evolution.

Population {B} is intermediate between population {A} and {C} because it shares different traits with each bounding population, traits that the other bounding population does not share, while sharing traits common to all three.

By definition any organism that meets this criteria is an intermediate variety, a representative of the transition from {A} to {C}:

Population {B} is transitional if it shares some traits with {A} that are not shared with {C} AND it shares some traits with {C} that are not shared with {A} AND it shares more traits with both {A} and {C}.

This is normal everyday observed and verified evolution by the change in hereditary traits in populations from generation to generation.

What this transition of form from {A} through {B} to {C} predicts is that IF this same pattern holds true for the past, that THEN we will see fossils that exhibit this pattern

What a transitional fossil is ...

By analogy from observing existing organisms:

Fossil {B} is transitional if it shares some traits with fossil {A} that are not shared with fossil\organism {C} AND it shares some traits with fossil\organism {C} that are not shared with fossil {A} AND it shares more traits with both fossil {A} and fossil\organism {C}.

We do not predict that you will find evidence of the "- (evolutionary path) - " nor does evolution predict that all {B} will be found, all that is predicted is that IF a fossil is found that it will show transitional characteristics, traits between ancestors and descendants as well as shared traits with ancestors, shared traits with descendants and shared traits with ancestors and descendants and many shared traits with both.

Thus from some ancestral shrew-like animal 65 million years ago, similar to this {A}:



... to the 54 million year old "new" bat fossil {B}:



... to this modern bat skeleton {C}



We can see that {B} is indeed intermediate between {A} and {C} but that more characteristics are shared by all three than are different between {A} and {C} AND that {B} shares some traits with {A} that it does NOT share with {C} (four claws on front arms, long tail, no echolocation) and it shares some traits with {C} that it does NOT share with {A} (long fingers being the most evident}.

Thus {B} is an intermediate fossil along the transition from non-bat to bat.